Base station apparatus and transmission method

ABSTRACT

The present disclosure provides a method of generating codebook in a wireless communication system with multiple antenna arrays, as well as a wireless communication system, base station and terminal using the codebook for communication. The method comprises steps of: providing a basic codebook which contains multiple pre-coding matrices; and assigning phase offsets to certain pre-coding matrices in the basic codebook to form a codebook with phase offset. The feedback overhead from a client to a base station side is reduced and a good precision of feedback for multi-antenna array is kept by applying the method of generating codebook and using the generated codebook in the wireless communication system, base station and terminal.

BACKGROUND

1. Technical Field

The present disclosure generally relates to the field of communication,and especially to a method, a base station and a terminal for generatingcodebook as well as a wireless communication system comprising the basestation and the terminal using the generated codebook.

2. Description of the Related Art

The multi-antenna technology is an important component of the fourthgeneration wireless communication system. An important application inthe multi-antenna technology is closed-loop downlink pre-coding. In thisapplication, when signals are sent to a client by multiple antennaslocated in a base station, the client measures a downlink transmissionchannel from the base station to the client, and suggests acorresponding pre-coding matrix to the base station according to thechannel condition. This suggestion is transmitted to the base stationvia a feedback link. Subsequently, in the downlink transmission, thebase station may adopt directly the suggested pre-coding matrix topre-code the signal to be sent to the client.

In the above procedure for closed-loop downlink pre-coding, an importantissue is how to feed back the pre-coding matrix suggested by the client.The means which is most popular and possibly most practical is a mannerof codebook based feedback. In such a feedback manner, a codebookcontaining multiple pre-coding matrices is first defined and is appliedat both the base station and the client. In this way, when suggesting acertain pre-coding matrix in the codebook to the base station accordingto the channel condition, the client can feed back the indexcorresponding to the pre-coding matrix to the base station. Forinstance, a codebook containing four pre-coding matrices, each of whichis indexed respectively as {00}, {01}, {10} and {11}, is known at boththe base station and the client. If the client suggests using the firstpre-coding matrix, the bits {00} are fed back to the base station. Afeedback manner based on the pre-coding matrix index can achieveimprovement in both reducing the feedback overhead and assuring a highdownlink throughput.

A basic codebook design is based on feedback of a pre-coding matrix fora single antenna array. However, in the current LTE-A (Long TermEvolution-Advanced) standardization, a client may receive signals frommultiple antenna arrays located in multiple base stations, for example,in downlink CoMP (Coordinate Multiple Points) as shown in FIG. 1. Insuch a case, a suggested pre-coding matrix needs to be fed back for eachantenna array, and thus multiple pre-coding matrices need to be fed backfor the multiple antenna arrays. In addition, in order to enable signalsfrom the multiple antenna arrays be combined coherently at the client,the phase offsets between the pre-coding matrices need to be fed back tothe base station as well.

With respect to the feedback manner of the pre-coding matrices under theabove downlink CoMP environment, for simplification, as an example,assuming that the base station 1 and the base station 2 are involved inthe downlink transmission, and each of the base stations uses an antennaarray containing two antennas for transmitting signals (as shown FIG.2), the description of a currently known feedback manner is made in thefollowing.

For the downlink CoMP environment as shown in FIG. 2, a manner of directfeedback at present is to use a two-antenna codebook defined in LTEstandard Rel-8 version for both the base station 1 and the base station2, i.e.,

$\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & j & {- j}\end{bmatrix}\text{/}{\sqrt{2}.}$

There are four pre-coding matrices

$\begin{bmatrix}1 \\1\end{bmatrix},\begin{bmatrix}1 \\{- 1}\end{bmatrix},{\begin{bmatrix}1 \\j\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}1 \\{- j}\end{bmatrix}}}$

included in the two-antenna codebook.Therefore, when the client performs feedback, two bits are used to feedback PMI1 (Pre-coding Matrix Index) suggested to be employed for theantenna array located in the base station 1, and two bits are used tofeed back PMI2 suggested to be employed for the antenna array located inthe base station 2 as well. Furthermore, one bit is additionallyrequired to feed back the phase offset (−1 or 1) between the PMI1 andthe PMI2 to the base station.

In the above feedback manner, five bits in total are used for feedback.In some cases, however, people may expect to reduce the feedbackoverhead. In such cases, two possible feedback manners are known asfollows.

The first possible manner is substantially the same as the abovefive-bit feedback manner, and the difference is that the one bit forfeeding back the phase offset between the PMI1 and the PMI2 is removed,that is, the phase offset is not fed back. In this way, only four bitsin total are needed for feedback. However, since the phase offset is notfed back, the signals transmitted from the antenna arrays of the basestations 1 and 2 may not be combined coherently at the client, thusresulting in the reduction of the system throughput.

Another possible manner is to reduce the number of bits for a pre-codingmatrix index (for example, PMI2) from two bits to one bit, and meanwhileone bit is still used to feed back the phase offset. Specifically, thetwo-antenna codebook defined in the LTE standard Rel-8 version, that is

${\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & j & {- j}\end{bmatrix}\text{/}\sqrt{2}},$

is still used for the base station 1, while a two-antenna codebookcontaining two pre-coding matrices

$\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}\text{/}\sqrt{2}$

is used for the base station 2. In this way, when the client is feedingback, two bits are still required to feed back the PMI1 for the antennaarray located in the base station 1, while only one bit is required tofeed back the PMI2 for the antenna array located in the base station 2.In addition, one bit is used to feed back the phase offset between thePMI1 and the PMI2 to the base stations. Thereby, the feedback overheadis reduced to four bits. However, the method results in reduction of thespatial granularity of the codebook of the PMI2, and may also causereduction of the throughput.

Thus, it has become an issue of the art how to reduce the feedbackoverhead while keeping the precision of feedback for a multi-antennaarray.

BRIEF SUMMARY

In order to address the above issues, the present disclosure provides amethod of generating codebook in a wireless communication system, awireless communication system, a base station, and a wireless terminalby using the method.

In the present disclosure, in order to show relationships betweenrespective elements of codebook more distinctly, the respective elementsof codebook may be also represented by their corresponding phases, andenable the amplitudes of all elements to be “1”.

According to one aspect of the present disclosure, there is provided amethod of generating codebook in a wireless communication system with aplurality of antenna arrays, comprising steps of: providing a basiccodebook which contains a plurality of pre-coding matrices; andassigning phase offsets to certain pre-coding matrices in the basiccodebook to form a codebook with phase offset.

According to another aspect of the present disclosure, there is provideda wireless communication system by using a plurality of codebooks forcommunication, the wireless communication system including a pluralityof base stations and a terminal performing downlink CoMP (CoordinateMultiple Points) transmission, each of the plurality of base stationsbeing configured with one antenna array, and a plurality of antennaarrays configured in the plurality of base stations respectivelycorresponding to the plurality of codebooks. The base station comprises:a feedback information reception unit for receiving information fed backfrom the terminal; a codebook conservation unit for forming a codebookwith phase offset by assigning phase offsets to certain pre-codingmatrices in a basic codebook, or for storing the basic codebook and thecodebook with phase offset in advance; and a sender unit for pre-codingdata to be transmitted, and sending the pre-coded data to the terminalvia the antenna array configured in the base station. The terminalcomprises: a codebook conservation unit for forming a codebook withphase offset by assigning phase offsets to certain pre-coding matricesin a basic codebook, or for storing the basic codebook and the codebookwith phase offset in advance; and an information feedback unit forfeeding back indexes identifying the pre-coding matrices in the basiccodebook and the codebook with phase offset to the plurality of antennaarrays in the plurality of base stations.

According to a further aspect of the present disclosure, there isprovided a base station performing downlink CoMP transmission,comprising: a feedback information reception unit for receivinginformation fed back from a terminal; a codebook conservation unit forforming a codebook with phase offset by assigning phase offsets tocertain pre-coding matrices in a basic codebook, or for storing thebasic codebook and the codebook with phase offset in advance; and asender unit for pre-coding data to be transmitted, and sending thepre-coded data to the terminal via the antenna array of the basestation.

In the above base station and the wireless communication systemaccording to the present disclosure, the antenna arrays in each of basestations performing downlink CoMP transmission contain the same numberof antennas, and a first antenna array located in a first base stationuses the basic codebook, while other antenna arrays located in otherbase stations use the codebook with phase offset.

In the above base station and the wireless communication systemaccording to the present disclosure, each of the antenna arrays containstwo antennas, and the basic codebook is a two-antenna codebook definedin the LTE standard Rel-8 version.

In the above base station and the wireless communication systemaccording to the present disclosure, each of the antenna arrays containsfour antennas, and the basic codebook is a four-antenna codebook definedin the LTE standard Rel-8 version.

In the above base station and the wireless communication systemaccording to the present disclosure, a first antenna array in a firstbase station of the multiple base stations performing downlink CoMPtransmission contains two antennas, while other antenna arrays in otherbase stations all contain four antennas. The basic codebook is afour-antenna codebook defined in the LTE standard Rel-8 version, and thecodebook with phase offset is used for the other antenna arrays, while atwo-antenna codebook defined in the LTE standard Rel-8 version is usedfor the first antenna array.

In the above base station and the wireless communication systemaccording to the present disclosure, a first antenna array in a firstbase station of the multiple base stations performing downlink CoMPtransmission contains four antennas, while other antenna arrays in otherbase stations each contain two antennas. The basic codebook is atwo-antenna codebook defined in the LTE standard Rel-8 version, andwherein the codebook with phase offset is used for the other antennaarrays, while a four-antenna codebook defined in the LTE standard Rel-8version is used for the first antenna array.

In the above base station and the wireless communication systemaccording to the present disclosure, the basic codebook is a DiscreteFourier Transform based three-bit four-antenna codebook:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135}\end{bmatrix},$

and the codebook with phase offset is

$\begin{bmatrix}0 & 180 & 0 & 180 & 0 & 180 & 0 & 180 \\0 & {- 135} & 90 & {- 45} & 180 & 45 & {- 90} & 135 \\0 & {- 90} & 180 & 90 & 0 & {- 90} & {- 180} & 90 \\0 & {- 45} & 270 & {- 135} & 180 & 135 & {- 270} & 45\end{bmatrix}.$

In the above base station according to the present disclosure, the basiccodebook is a four-antenna codebook defined in the LTE standard Rel-8version:

$\begin{bmatrix}{0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & 0 \\{0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & 180\end{bmatrix},$

and the codebook which phase offset is

$\begin{bmatrix}{0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & 0 \\{0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & 0 \\{0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & 0\end{bmatrix}.$

According to a further aspect of the present disclosure, there isprovided a wireless communication system by using a codebook forcommunication, the wireless communication system including a basestation and a terminal, the base station being configured with twoantenna arrays. The base station comprises: a feedback informationreception unit for receiving information fed back from the terminal; acodebook conservation unit for forming a codebook with phase offset byassigning phase offsets to certain pre-coding matrices in a basiccodebook, or for storing a codebook obtained by combination of the basiccodebook and the codebook with phase offset in advance, wherein the twoantenna arrays correspond to the basic codebook and the codebook withphase offset respectively; and a sender unit for pre-coding data to betransmitted, and sending the pre-coded data to the terminal via theantenna arrays. The terminal comprises: a codebook conservation unit forforming a codebook with phase offset by assigning phase offsets tocertain pre-coding matrices in a basic codebook, or for storing thebasic codebook and the codebook with phase offset in advance; and aninformation feedback unit for feeding back indexes identifying thepre-coding matrices in the basic codebook and the codebook with phaseoffset to the antenna arrays in the base station.

According to a further aspect of the present disclosure, there isprovided a base station by using a codebook for communication, the basestation being configured with two antenna arrays, the base stationcomprising: a feedback information reception unit for receivinginformation fed back from a terminal; a codebook conservation unit forforming a codebook with phase offset by assigning phase offsets tocertain pre-coding matrices in a basic codebook, or for storing acodebook obtained by combination of the basic codebook and the codebookwith phase offset in advance, wherein the two antenna arrays correspondto the basic codebook and the codebook with phase offset respectively;and a sender unit for pre-coding data to be transmitted, and sending thepre-coded data to the terminal via the antenna arrays.

In the above base station according to the present disclosure, the basiccodebook is a Discrete Fourier Transform based three-bit four-antennacodebook:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135}\end{bmatrix},$

and the codebook with phase offset is

$\begin{bmatrix}0 & 180 & 0 & 180 & 0 & 180 & 0 & 180 \\0 & {- 135} & 90 & {- 45} & 180 & 45 & {- 90} & 135 \\0 & {- 90} & 180 & 90 & 0 & {- 90} & {- 180} & 90 \\0 & {- 45} & 270 & {- 135} & 180 & 135 & {- 270} & 45\end{bmatrix}.$

In the above base station according to the present disclosure, the basiccodebook is a four-antenna codebook defined in the LTE standard Rel-8version:

$\begin{bmatrix}{0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & 0 \\{0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & 180\end{bmatrix},$

and the codebook which phase offset is

$\quad{\begin{bmatrix}{0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\;} & 180 & 180 \\{0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {180\;} & 0 & 0 \\{0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\;} & 180 & 0 \\{0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {180\;} & 180 & 0\end{bmatrix}.}$

According to a further aspect of the present disclosure, there isprovided a wireless terminal, comprising: a codebook conservation unitfor forming a codebook with phase offset by assigning phase offsets tocertain pre-coding matrices in a basic codebook, or for storing thebasic codebook and the codebook with phase offset in advance; and aninformation feedback unit for feeding back indexes identifying thepre-coding matrices in the basic codebook and the codebook with phaseoffset to at least one base station.

In the above wireless terminal according to the present disclosure, thebasic codebook is a two-antenna codebook defined in the LTE standardRel-8 version.

In the above wireless terminal according to the present disclosure, thebasic codebook is a four-antenna codebook defined in the LTE standardRel-8 version.

In the above wireless terminal according to the present disclosure, theantenna arrays in each of base stations performing downlink CoMPtransmission contain the same number of antennas, and a first antennaarray located in a first base station uses the basic codebook, whileother antenna arrays located in other base stations use the codebookwith phase offset.

In the above wireless terminal according to the present disclosure, eachof the antenna arrays contains two antennas, and the basic codebook is atwo-antenna codebook defined in the LTE standard Rel-8 version.

In the above wireless terminal according to the present disclosure, eachof the antenna arrays contains four antennas, and the basic codebook isa four-antenna codebook defined in the LTE standard Rel-8 version.

In the above wireless terminal according to the present disclosure, afirst antenna array in a first base station of the multiple basestations performing downlink CoMP transmission contains two antennas,while other antenna arrays in other base stations all contain fourantennas. The basic codebook is a four-antenna codebook defined in theLTE standard Rel-8 version, and the codebook with phase offset is usedfor the other antenna arrays, while a two-antenna codebook defined inthe LTE standard Rel-8 version is used for the first antenna array.

In the above wireless terminal according to the present disclosure, afirst antenna array in a first base station of the multiple basestations performing downlink CoMP transmission contains four antennas,while other antenna arrays in other base stations each contain twoantennas. The basic codebook is a two-antenna codebook defined in theLTE standard Rel-8 version, and wherein the codebook with phase offsetis used for the other antenna arrays, while a four-antenna codebookdefined in the LTE standard Rel-8 version is used for the first antennaarray.

In the above wireless terminal according to the present disclosure, thebasic codebook is a four-antenna codebook defined in the LTE standardRel-8 version, wherein respective base stations of the base station sideare all configured with one antenna array, and the information feedbackunit feeds back an index identifying a pre-coding matrix in atwo-antenna codebook defined in the LTE standard Rel-8 version to afirst antenna array in a first base station of the base station side,and feeds back indexes identifying the pre-coding matrices in thecodebook with phase offset to other antenna arrays in other basestations other than the first base station of the base station side.

In the above wireless terminal according to the present disclosure, thebasic codebook is a two-antenna codebook defined in the LTE standardRel-8 version, wherein respective base stations of the base station sideare all configured with one antenna array, and the information feedbackunit feeds back an index identifying a pre-coding matrix in afour-antenna codebook defined in the LTE standard Rel-8 version to afirst antenna array in a first base station of the base station side,and feeds back indexes identifying the pre-coding matrices in thecodebook with phase offset to other antenna arrays in other basestations other than the first base station of the base station side.

In the above wireless terminal according to the present disclosure, thebasic codebook is a Discrete Fourier Transform based three-bitfour-antenna codebook:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135}\end{bmatrix},$

and the codebook with phase offset is

$\begin{bmatrix}0 & 180 & 0 & 180 & 0 & 180 & 0 & 180 \\0 & {- 135} & 90 & {- 45} & 180 & 45 & {- 90} & 135 \\0 & {- 90} & 180 & 90 & 0 & {- 90} & {- 180} & 90 \\0 & {- 45} & 270 & {- 135} & 180 & 135 & {- 270} & 45\end{bmatrix}.$

In the above wireless terminal according to the present disclosure, thebasic codebook is a four-antenna codebook defined in the LTE standardRel-8 version:

$\begin{bmatrix}{0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\;} & {0\mspace{11mu}} & {0\;} & {0\mspace{11mu}} & {0\mspace{11mu}} & 0 \\{0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\;} & {{- 90}\mspace{11mu}} & {0\;} & {0\mspace{11mu}} & {180\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\;} & {0\mspace{11mu}} & {0\;} & {180\mspace{11mu}} & {0\mspace{11mu}} & 180 \\{0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\;} & {{- 90}\mspace{11mu}} & {180\;} & {0\mspace{11mu}} & {0\mspace{11mu}} & 180\end{bmatrix},$

and the codebook which phase offset is

$\quad{\begin{bmatrix}0 & 0 & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & 180 \\0 & 90 & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {45\mspace{11mu}} & {135\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & 0 \\0 & 180 & {0\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {{- 90}\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & 0 \\0 & {- 90} & {180\mspace{11mu}} & {90\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {135\mspace{11mu}} & {45\mspace{11mu}} & {180\mspace{11mu}} & {90\mspace{11mu}} & {0\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {180\mspace{11mu}} & 0\end{bmatrix}.}$

According to a further aspect of the present disclosure, there isprovided a method of generating codebook in a wireless communicationsystem with a plurality of antenna arrays, comprising steps of:providing a basic codebook which contains two sets of base vectors,respective base vectors of each set of base vectors being orthogonalwith each other; acquiring vectors being orthogonal with at least onebase vector in a second set of base vectors; making a vector whosecorrelation with one base vector of a first set of base vectors is thehighest among the acquired vectors be a new base vector; generatingother new base vectors; and replacing the base vectors with the new basevectors respectively to form a new basic codebook.

According to a further another aspect of the present disclosure, thereis provided a method of generating a codebook in a wirelesscommunication system, the method comprising: a setting step of setting abasic codebook comprising a plurality of base vectors which are groupedinto a plurality of sets, respective base vectors in each set of basevectors being orthogonal with each other; a computation step ofcomputing a new base vector corresponding to one base vector in one setof the plurality of sets of base vectors, so that the new base vectorequals to another base vector in other sets of base vectors other thanthe one set of base vectors, and repeating the computation step toacquire a plurality of new base vectors corresponding to the pluralityof base vectors; and a generation step of generating a new basiccodebook using the respective new base vectors.

In the above method according to the present disclosure, the pluralityof base vectors in the basic codebook are w_(m), which are DFT vectors,and differences between directions of base vectors w_(m) and w_(m+n) aren times of 11.25° in turn, where m and n are integer numbers infollowing embodiments and claims, although they are also possible to bereal numbers.

In the above method according to the present disclosure, the obtainednew base vectors are w′_(m). w′_(m) is also DFT vectors but thedirections of which are different from the corresponding base vectorsw_(m). Wherein w′_(m)=w_(m+n), for example, it is possible to havew′_(m)=w_(m+1), or w′_(m)=w_(m+2), or w′_(m)=w_(m+3), etc.

In the above method according to the present disclosure, the form of arank 2 pre-coding vector in the new basic codebook is

$\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}$ ${{and}\text{/}{{or}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{j\; w_{m}} & {{- j}\; w_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}$ ${and}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}$

are used to precode the signals in different layers sent from a basestation respectively,

$\begin{bmatrix}w_{m} \\{j\; w_{m}}\end{bmatrix}$ ${and}\begin{bmatrix}w_{m}^{\prime} \\{{- j}\; w_{m}^{\prime}}\end{bmatrix}$

are used to precode the signals in different layers sent from the basestation respectively.

According to a further another aspect of the present disclosure, thereis provided a base station for performing communication using acodebook, the base station comprising: a feedback information receptionunit for receiving information fed back from a terminal; a codebookconservation unit for saving a basic codebook and/or a new basiccodebook, the basic codebook comprising a plurality of base vectorswhich are grouped into a plurality of sets, respective base vectors ineach set of base vectors being orthogonal with each other, the new basiccodebook being obtained in a way of: computating a new base vectorcorresponding to one base vector in one set of the plurality of sets ofbase vectors, so that the new base vector equals to another base vectorin other sets of base vectors other than the one set of base vectors,and repeating the above operation to acquire a plurality of new basevectors corresponding to the plurality of base vectors, and generatingthe new basic codebook using the respective new base vectors; and asender unit for pre-coding data to be transmitted by using the basiccodebook and/or the new basic codebook based on the information fed backfrom the terminal, and sending the pre-coded data to the terminal via anantenna array.

In the above base station according to the present disclosure, theplurality of base vectors in the basic codebook are w_(m), which are DFTvectors, and differences between directions of base vectors w_(m) andw_(m+n) are n times of 11.25° in turn, where m and n are integer numbersin following embodiments and claims, although they are also possible tobe real numbers.

In the above base station according to the present disclosure, theobtained new base vectors are w′_(m). w′_(m) is also DFT vectors but thedirections of which are different from the corresponding base vectorsw_(m). Wherein w′_(m)=w_(m+n), for example, it is possible to havew′_(m)=w_(m+1), or w′_(m)=w₃₊₂, or w′_(m)=w_(m+3), etc.

In the above base station according to the present disclosure, the formof a rank 2 pre-coding vector in the new basic codebook is

$\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}$ ${{and}\text{/}{{or}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{j\; w_{m}} & {{- j}\; w_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}$ ${and}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}$

are used to precode the signals in different layers sent from a basestation respectively,

$\begin{bmatrix}w_{m} \\{j\; w_{m}}\end{bmatrix}$ ${and}\begin{bmatrix}w_{m}^{\prime} \\{{- j}\; w_{m}^{\prime}}\end{bmatrix}$

are used to precode the signals in different layers sent from the basestation respectively.

According to a further another aspect of the present disclosure, thereis provided a terminal for performing communication using a codebook,the terminal comprising: a codebook conservation unit for saving a basiccodebook and/or a new basic codebook, the basic codebook comprising aplurality of base vectors which are grouped into a plurality of sets,respective base vectors in each set of base vectors being orthogonalwith each other, the new basic codebook being obtained in a way of:computating a new base vector corresponding to one base vector in oneset of the plurality of sets of base vectors, so that the new basevector equals to another base vector in other sets of base vectors otherthan the one set of base vectors, and repeating the above operation toacquire a plurality of new base vectors corresponding to the pluralityof base vectors, and generating the new basic codebook using therespective new base vectors; and a information feedback unit for feedingback indexes identifying pre-coding matrixes in the basic codebookand/or the new basic codebook to a base station.

In the above terminal according to the present disclosure, the pluralityof base vectors in the basic codebook are w_(m), which are DFT vectors,and differences between directions of base vectors w_(m) and w_(m+n) aren times of 11.25° in turn, where m and n are integer numbers infollowing embodiments and claims, although they are also possible to bereal numbers.

In the above terminal according to the present disclosure, the obtainednew base vectors are w′_(m). w′_(m) is also DFT vectors but thedirections of which are different from the corresponding base vectorsw_(m). Wherein w′_(m)=w_(m+n), for example, it is possible to havew′_(m)=w_(m+1), or w′_(m)=w_(m+2), or w′_(m)=w_(m+3), etc.

In the above terminal according to the present disclosure, the form of arank 2 pre-coding vector in the new basic codebook is

${\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}\mspace{14mu} {and}\text{/}{{or}\mspace{14mu}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{jw}_{m} & {- {jw}_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from a basestation respectively, and

$\begin{bmatrix}w_{m} \\{jw}_{m}\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}w_{m}^{\prime} \\{- {jw}_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from the basestation respectively.

By applying the method of generating a codebook with phase offset from abasic codebook provided by the present disclosure, and by using thegenerated codebook in a wireless communication system, base stations andterminals, the phase offsets between the antenna arrays may be fed backwithout increasing the size of the codebook, and the spatial granularityof the codebook is not reduced. Therefore, compared to the methodsemployed in the prior art, the present disclosure can effectivelyimprove the precision of feedback for a multi-antenna array with thesame feedback overhead.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and/or other aspects and advantages of the present disclosure willbecome more distinct and easier to be understand from a detaileddescription of embodiments of the present disclosure in combination withattached drawings below, in which:

FIG. 1 shows an exemplary systematic framework for downlink CoMPtransmission system;

FIG. 2 shows a schematic diagram for applying a codebook generationmethod of an embodiment of the present disclosure under an environmentin which two base stations take part in a collaborative downlinktransmission and each of the base stations employs an antenna arraycontaining two antennas to perform signal transmission;

FIG. 3 shows a flow chart of a codebook generation method according to afirst embodiment of the present disclosure;

FIG. 4 shows a schematic diagram for applying a codebook generationmethod of an embodiment of the present disclosure under an environmentin which two base stations take part in a collaborative downlinktransmission and each of the base stations employs an antenna arraycontaining four antennas to perform signal transmission;

FIG. 5 shows a schematic diagram for applying a codebook generationmethod of an embodiment of the present disclosure under an environmentin which multiple base stations take part in a collaborative downlinktransmission and each of the base stations employs an antenna arraycontaining two antennas to perform signal transmission;

FIG. 6 shows a configuration for eight dual-polarized antennas;

FIG. 7 shows a structural block diagram of a base station using acodebook generated according to the codebook generation method of thefirst embodiment of the present disclosure to perform communication;

FIG. 8 shows a structural block diagram of a wireless terminal using acodebook generated according to the codebook generation method of thefirst embodiment of the present disclosure to perform communication;

FIG. 9 shows an exemplary system of a multi-user transmission system;

FIG. 10 shows the grouped orthogonal property of base vectors in a DFTbased three-bit four-antenna codebook;

FIG. 11 shows the orthogonal property of the pre-coding vectorcorresponding to PMI0 in a four-bit codebook for eight dual-polarizedantennas according to a sixth embodiment;

FIG. 12 shows the orthogonal property of the pre-coding vectorcorresponding to PMI0 in a four-bit codebook for eight dual-polarizedantennas according to an eighth embodiment; and

FIG. 13 is a flow chart of a method of generating codebook in a wirelesscommunication system of multiple antenna arrays according to anotherembodiment of the present disclosure.

DETAILED DESCRIPTION

Some specific embodiments of the present disclosure will be described indetail in combination with attached drawings below. If the detaileddescription of some related prior art may confuse the main points of thedisclosure, the detailed description thereof will not be provided here.In respective embodiments, identical reference numerals are used todenote elements or units performing the same functions.

First Embodiment

In the present embodiment, by employing a novel codebook design in whichphase offsets and pre-coding matrix indexes are fed back simultaneously,the feedback overhead is reduced and a good precision of feedback formulti-antenna array is kept.

First, a specific implementation provided by the present disclosure withrespect to the above five-bit feedback manner, whose performance is thebest, is introduced. According to the specific embodiment, for thedownlink CoMP environment as shown in FIG. 2, the above two-bittwo-antenna codebook of Rel-8, that is

${\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & j & {- j}\end{bmatrix}/\sqrt{2}},$

is employed for any one of base stations (for example, the base station1), while a novel three-bit two-antenna codebook with phase offset,i.e.,

$\begin{bmatrix}1 & 1 & 1 & 1 & {- 1} & {- 1} & {- 1} & {- 1} \\1 & {- 1} & j & {- j} & {- 1} & 1 & {- j} & j\end{bmatrix}/\sqrt{2}$

is used for the other base station (for example, the base station 2). Itcan be seen that the three-bit two-antenna codebook is composed of thefour pre-coding matrices in the codebook for the base station 1 and fourcorresponding pre-coding matrices obtained by applying the phase offsetof −1 to the four pre-coding matrices. In this way, when the client isfeeding back, two bits will be used to feed back the index for thepre-coding matrix suggested to be employed for the antenna array locatedin the base station 1, while three bits will be used to feed back forthe antenna array located in the base station 2. Apparently, in thethree-bit feedback, not only the pre-coding matrix suggested to beemployed but also the phase offset is fed back.

Firstly it is observed that there are highly correlated pre-codingmatrices in the two antenna codebook of LTE Rel-8. For example, thecorrelation of [1 1] and [1 j] is high, and the correlation of [1 −1]and [1 −j] is high. Moreover it is observed that the correlation ofcombined pre-coding matrices [1 1 1 1] and [1 1 1 j] of the two basestations is very high when it is assumed that the pre-coding matrix forthe antenna array in the base station 1 has been determined to be [1 1].Thus, if the feedback overhead is required to be reduced, one of [1 1]and [1 j] in the three-bit codebook may be deleted. Similarly, thecorrelation of combined pre-coding matrices [1 1 −1 −1] and [1 1 −1 −j]of the two base stations is also very high, so that one of [−1 −1] and[−1 −j] in the three-bit codebook may be deleted to reduce the feedbackoverhead. At last, it is chosen to delete [1 j] and [−1 −1] for thepurpose of securing the spatial granularity of codebook for the antennaarray of the base station 2. For the same reason, [1 −j] and [−1 1] inthe three-bit codebook can be further deleted to reduce the feedbackoverhead.

Finally, the above three-bit codebook with the phase offset can besimplified as a two-bit codebook

$\begin{bmatrix}1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- j} & j\end{bmatrix}/{\sqrt{2}.}$

In this way, when the client is feeding back, only two bits are requiredto feed back for both the base station 1 and the base station 2respectively, so that the feedback overhead is reduced. On the otherhand, since the two-bit codebook for the base station 2 contains thephase offset, the phase offset is reflected in the feedback from theclient to the base station, so that signals transmitted from the basestation 1 and the base station 2 can be combined coherently at theclient.

It is worth mentioning that [1 j], [−1 −1], [1 −j] and [−1 1] aredeleted from the three-bit codebook to reduce the feedback overhead inthe above procedure. However, it is only one exemplary choice fordeletion. For example, for the combined pre-coding matrices [1 1 1 1]and [1 1 1 j] having high correlation, [1 j] is chosen to be deletedamong [1 1] and [1 j] in the three-bit codebook in the above example,but actually, it is possible to choose to delete [1 1]. Similarly, forthe combined pre-coding matrices [1 1 −1 −1] and [1 1 −1 −j] having highcorrelation, [−1 −j] may also be deleted among [−1 −1] and [−1 −j] inthe three-bit codebook. By such different choices for deletion,different simplified two-bit codebook can be finally obtained from thethree-bit codebook. For example, a two-bit codebook

$\begin{bmatrix}{- 1} & {- 1} & 1 & 1 \\{- 1} & 1 & j & {- j}\end{bmatrix}/\sqrt{2}$

is obtained by deleting [1 1], [−1 −j], [1 −1] and [−1 j] from thethree-bit codebook. The two-bit codebook may replace the abovesimplified codebook

$\begin{bmatrix}1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- j} & j\end{bmatrix}/\sqrt{2}$

as the codebook for the antenna array in the base station 2.

A specific procedure for deduction in which a simplified two-bittwo-antenna codebook is obtained from a three-bit two-antenna codebookhas been described above, on the basis of which the present disclosuresummarizes a general method of generating a two-bit two-antenna codebookwith phase offset as a target. According to the method, a codebook withphase offset for the antenna array in the base station 2 can be obtaineddirectly from a basic codebook

$\begin{bmatrix}1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- j} & j\end{bmatrix}/\sqrt{2}$

for the antenna array in the base station 1.

FIG. 3 shows a method of generating a codebook with phase offset from abasic codebook according to the present embodiment. As shown in FIG. 3,at step S301, the pre-coding matrix pairs having high correlations inthe basic codebook are determined; and subsequently at step S302, foreach one of the pre-coding matrix pairs, a phase offset of −1 isassigned to the two pre-coding matrices therein. Thereby, a codebookwith phase offset in which the phase offset and the pre-coding matrixmay be fed back simultaneously can be obtained. It should be noted thatthe meanings and definition of the high correlation as described aboveis well known for those skilled in the art, and it will not beillustrated in details here any more. According to the method, for theabove downlink CoMP environment as shown in FIG. 2, the pre-codingmatrix pairs having high correlations, that is [1 1] and [1 j] as wellas [1 −1] and [1 −], are firstly determined in the basic codebook

$\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & j & {- j}\end{bmatrix}/\sqrt{2}$

for the antenna array in the base station 1. Then, for the pre-codingmatrix pair [1 1] and [1 j] therein, a phase offset of −1 is applied tofor example [1 j] to obtain [−1 −j]; also, for the pre-coding matrixpair [1 −1] and [1 −j] therein, a phase offset of −1 is applied to forexample [1 −j] to obtain [−1 j]. Thereby, a new codebook

$\begin{bmatrix}1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- j} & j\end{bmatrix}/\sqrt{2}$

is obtained from the basic codebook, which is a simplified two-bittwo-antenna codebook obtained by deleting [1 j], [−1 −1], [1 −j] and [−11] from the three-bit codebook as described above.

It is easy to understand that when a phase offset of −1 is assigned totwo pre-coding matrices in a pre-coding matrix pair having a highcorrelation according to the step S302, the phase offset of −1 can beapplied to either of the pre-coding matrices. For example, in the aboveexemplary illustration, for the pre-coding matrix pair [1 1] and [1 j],a phase offset of −1 is applied to [1 j], and practically, the phaseoffset of −1 may be applied to [1 1] as well and thus [−1 −1] isobtained. Also, for the pre-coding matrix pair [1 −1] and [1 −j], aphase offset of −1 may also be applied to for example [1 −1] to obtain[−1 1]. Thereby, a new codebook

${\begin{bmatrix}{- 1} & {- 1} & 1 & 1 \\{- 1} & 1 & j & {- j}\end{bmatrix}/\sqrt{2}},$

that is a simplified two-bit two-antenna codebook obtained by deleting[1 1], [−1 −j], [1 −1] and [−1 j] from the three-bit codebook asdescribed above, is obtained from the basic codebook.

It is worth mentioning that although, for each pre-coding matrix pair, aphase offset of −1 is assigned to the two pre-coding matrices in thestep S302 of the above method according to the present embodiment, theassignment of the phase offset of −1 is only one simplest manner ofphase offset assignment. In fact, a phase offset may be assigned to twopre-coding matrices included in a pre-coding matrix pair according towhether the correlation of the two pre-coding matrices is high or low.For example, one advantageous generalization is to, for each pre-codingmatrix pair, assign a phase offset to the two pre-coding matricesincluded therein according to whether the correlation of the twopre-coding matrices is high or low, such that the higher the correlationof the two pre-coding matrices in the pre-coding matrix pair is, thelarger the assigned phase offset is.

Second Embodiment

In the first embodiment, taking a case in which the base station 1 andthe base station 2 take part in a collaborative downlink transmissionwith each of the base stations employing an antenna array containing twoantennas to perform signal transmission as an example, a method ofgenerating codebook according to one example of the present disclosureis described. Also, the present disclosure may be applied to a downlinkCoMP environment in which the antenna array of each base stationcontains other numbers of antennas. For example, in the presentembodiment, a method of generating codebook according to another exampleof the present disclosure is explained by taking a case in which a basestation 1 and a base station 2 take part in a collaborative downlinktransmission with each of the base stations employing an antenna arraycontaining four antennas to perform signal transmission (as shown inFIG. 4) as an example.

In the downlink CoMP environment as shown in FIG. 4, both of the basestation 1 and the base station 2 employ four transmission antennas toperform signal transmission to a client. Therefore, a Rel-8 four-antennacodebook is used for one of the base stations (for example the basestation 1), and for the other base station (for example the base station2), a four-antenna codebook with phase offset is used, which isgenerated according to the method of generating codebook of the firstembodiment of the present disclosure taking the Rel-8 four-antennacodebook as a basic codebook.

The Rel-8 four-antenna codebook is well known for those skilled in theart which contains 16 pre-coding matrices. According to the method ofgenerating codebook of the first embodiment of the present disclosure,the 16 pre-coding matrices in the codebook are grouped into 8 pre-codingmatrix pairs having high correlations. Subsequently, a phase offset isassigned to two pre-coding matrices of each of the pre-coding matrixpairs, thereby a codebook with phase offset is obtained. An example of aRel-8 four-antenna codebook and a codebook with phase offset which isgenerated from the Rel-8 four-antenna codebook are shown as follows. (inthe following, in order to show relationships between respectiveelements in a codebook more distinctly, the respective elements arerepresented by their corresponding phases, and amplitudes of all theelements are 1).

The Rel-8 four-antenna codebook is as follows:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 90 & 180 & {- 90} & 45 & 135 & {- 135} & {- 45} & 0 & 90 & 180 & {- 90} & 0 & 0 & 180 & 180 \\0 & 180 & 0 & 180 & 90 & {- 90} & 90 & {- 90} & 180 & 0 & 180 & 0 & 0 & 180 & 0 & 180 \\0 & {- 90} & 180 & 90 & 135 & 45 & {- 45} & {- 135} & 180 & 90 & 0 & {- 90} & 180 & 0 & 0 & 180\end{bmatrix}\quad$

By generating 8 pre-coding matrix pairs having high correlations from 16pre-coding matrices in the codebook in a pairing manner of {1, 5} {2, 6}{3, 7} {4, 8} {9, 13} {10, 14} {11, 15} {12, 16} (respective numbersrepresent sequence numbers for pre-coding matrices in a codebook) andassigning a phase offset of 180° (i.e., a phase offset of −1) to each ofpre-coding matrix pairs, a codebook with phase offset is generated asfollows:

$\begin{bmatrix}0 & 0 & 0 & 0 & 180 & 180 & 180 & 180 & 0 & 0 & 0 & 0 & 180 & 180 & 180 & 180 \\0 & 90 & 180 & {- 90} & {- 135} & {- 45} & 45 & 135 & 0 & 90 & 180 & {- 90} & 180 & 180 & 0 & 0 \\0 & 180 & 0 & 180 & {- 90} & 90 & {- 90} & 90 & 180 & 0 & 180 & 0 & 180 & 0 & 180 & 0 \\0 & {- 90} & 180 & 90 & {- 45} & {- 135} & 135 & 45 & 180 & 90 & 0 & {- 90} & 0 & 180 & 180 & 0\end{bmatrix}\quad$

It is to be explained that the Rel-8 four-antenna codebook given aboveis only a possible form, and the codebook may also be composed of otherpre-coding matrices as known by those skilled in the art.

On the other hand, the above manner of grouping 16 pre-coding matricesinto 8 pre-coding matrix pairs having high correlations is notexclusive, and other pairing manners may be employed according tocorrelations between respective matrices. In addition, although thephase offset of 180° is assigned to two pre-coding matrices in each ofpre-coding matrix pair in the above example, more generally a phaseoffset may be assigned according to the correlation between the twopre-coding matrices of a pre-coding matrix pair, as mentioned in thefirst embodiment, such that for example the higher the correlationbetween the two pre-coding matrices of the pre-coding matrix pair is,the larger the assigned phase offset is.

Third Embodiment

In the above two embodiments, taking the cases in which only two basestations perform collaborative downlink transmissions as examples,methods for codebook generation according to some examples of thepresent disclosure are described. The present disclosure may also beapplied to situations of multiple base stations taking part in CoMPdownlink transmission. In the following, a description of a method ofgenerating codebook according to one example of the present disclosureis made by taking a case as shown in FIG. 5 as an example, in whichmultiple base stations take part in collaborative downlink transmissionwith each of the base stations employing an antenna array containing twoantennas to perform signal transmission.

In the present embodiment, with respect to the downlink CoMP environmentas shown in FIG. 5, a Rel-8 two-bit two-antenna codebook, i.e.,

${\begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & j & {- j}\end{bmatrix}/\sqrt{2}},$

, is used for any one of the base stations (for example the base station1), and a codebook with phase offset which is generated according to themethod of generating codebook of the first embodiment of the presentdisclosure, for example

${\begin{bmatrix}1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- j} & j\end{bmatrix}\text{/}\sqrt{2}},$

is used for each of other base stations. It should be understood bythose skilled in the art that by applying a codebook with phase offsetwhich is generated according to the method of generating codebook of thefirst embodiment to each of the base stations other than the basestation 1, the phase offsets between the antenna array in each of theother base stations and the antenna array in the base station 1 can befed back.

It is easy to be understand that although the description is made takingthe case in which the antenna array in each of base stations containstwo antennas as an example in the present embodiment, the disclosure mayalso be applied to a downlink CoMP environment where the antenna arrayin each of base stations contains other numbers of antennas (for examplefour antennas).

Fourth Embodiment

In the above embodiments, by taking cases in which the antenna array ineach of base stations perform collaborative downlink transmissionscontains the same number of antennas as examples, the methods forcodebook generation according to some examples of the present disclosureare described. The present disclosure may also be applied to situationsin which the antenna array in each of base stations contains differentnumber of antennas, which will be described in the present embodiment.

For convenience of description, it is assumed that a certain basestation (for example the base station 1) of the multiple base stationsas shown in FIG. 5 uses an antenna array containing four antennas forsignal transmission, and each of other base stations still uses anantenna array containing two antennas for signal transmission. In suchan exemplary case, according to the present embodiment, a Rel-8four-antenna codebook is used for the base station 1, while atwo-antenna codebook with phase offset, which is generated according tothe first embodiment of the present disclosure by taking the Rel-8two-antenna codebook as a basic codebook, is used for each of other basestations. The Rel-8 four-antenna codebook and the two-antenna codebookhave been described in the foregoing embodiments, and unnecessarydetails thereof will not be given herein.

Similarly, it is assumed that a certain base station (for example thebase station 1) of the multiple base stations as shown in FIG. 5 stilluses an antenna array containing two antennas for signal transmission,and each of other base stations uses an antenna array containing fourantennas for signal transmission. In such an exemplary case, accordingto the present embodiment, a Rel-8 two-antenna codebook is used for thebase station 1, while a four-antenna codebook with phase offset, whichis generated according to the first embodiment of the present disclosureby taking a Rel-8 four-antenna codebook as a basic codebook, is used foreach of other base stations.

Fifth Embodiment

The above embodiments describe methods for codebook generation in CoMPenvironments according to one aspect of the present disclosure. In fact,the disclosure may also be applied in dual-polarized antennas.

FIG. 6 shows a configuration for eight dual-polarized antennas. As shownin FIG. 6, antennas 1, 2, 3 and 4 have the same polarization feature,that is polarization of positive 45 degree; antennas 5, 6, 7 and 8 havethe same polarization feature, that is polarization of negative 45degree; and antenna pairs of 1 and 5, 2 and 6, 3 and 7, 4 and 8 arerespectively at the same physical position. Since spaces betweendual-polarized antennas are generally half-wavelength, the antennas 1,2, 3 and 4 have correlation, and the antennas 5, 6, 7 and 8 havecorrelation. Meanwhile, the antenna array 1 containing the antennas 1,2, 3 and 4 and the antenna array 2 containing the antennas 5, 6, 7 and 8are independent from each other.

It can be seen from the above description that for dual-polarizedantennas, the antennas can be considered equivalently as two antennaarrays in a physical model. Therefore, the methods for codebookgeneration according to the above embodiments of the present disclosuremay be applied for the dual-polarized antennas. Specifically speaking,for the dual-polarized antennas, a basic codebook may be applied to thefirst antenna array containing antennas with a first same polarizationfeature (for example a polarization of positive 45 degree). Then, acodebook with phase offset is generated by applying the method ofgenerating codebook of the first embodiment of the present disclosure.The codebook with phase offset is used for the second antenna arraycontaining antennas with a second same polarization feature (for examplea polarization of negative 45 degree). Finally, the basic codebook forthe first antenna array and the generated codebook for the secondantenna array are combined into one codebook along the column directionas the codebook for dual-polarized antennas.

It should be explained that the pre-coding matrix index feedback in acase of dual-polarized antennas is different from that in a CoMP case.As described above, under a CoMP environment, certain bits are requiredto feed back the index for the pre-coding matrix suggested to beemployed for the antenna array in each of base stations. For example, inthe first embodiment, when the client performs feedback to the basestations, two bits will be used to feed back the index for thepre-coding matrix suggested to be employed by the antenna array in thebase station 1, meanwhile, other two bits will be used to feed back theindex for the pre-coding matrix suggested to be employed by the antennaarray in the base station 2. However, in a case of dual-polarizedantennas, the pre-coding matrix indexes for the first antenna array andthe second antenna array are highly correlated, thus the pre-codingmatrix index for the second antenna array is always the same as thepre-coding matrix index for the first antenna array. That is to say, inthis case, only the index for the pre-coding matrix suggested to be usedby the first antenna array is needed to be fed back, and there is noneed to use redundant bits to feed back the index for the pre-codingmatrix suggested to be employed by the second antenna array.

In the following, taking a case in which a DFT (Discrete FourierTransform) based three-bit four-antenna codebook is used as a basiccodebook for the first antenna array as an example, the application ofthe method of generating codebook according to the first embodiment ofthe present disclosure in an environment of eight dual-polarizedantennas is described.

The DFT based three-bit four-antenna codebook is well known for thoseskilled in the art, and a possible form thereof represented in phases is

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135}\end{bmatrix}.$

According to the method of generating codebook in the first embodimentof the present disclosure, the 8 pre-coding matrices in the abovecodebook are grouped into 4 pre-coding matrix pairs having highcorrelations, and a phase offset of 180° is assigned to each of thepre-coding matrix pairs, thereby generating a codebook with phase offsetfor the second antenna array as follows:

$\begin{bmatrix}0 & {0 + 180} & 0 & {0 + 180} & 0 & {0 + 180} & 0 & {0 + 180} \\0 & {45 + 180} & 90 & {135 + 180} & 180 & {{- 135} + 180} & {- 90} & {{- 45} + 180} \\0 & {90 + 180} & 180 & {{- 90} + 180} & 0 & {90 + 180} & {- 180} & {{- 90} + 180} \\0 & {135 + 180} & 270 & {45 + 180} & 180 & {{- 45} + 180} & {- 270} & {{- 135} + 180}\end{bmatrix},$

after simplification, it is

$\begin{bmatrix}0 & 180 & 0 & 180 & 0 & 180 & 0 & 180 \\0 & {- 135} & 90 & {- 45} & 180 & 45 & {- 90} & 135 \\0 & {- 90} & 180 & 90 & 0 & {- 90} & {- 180} & 90 \\0 & {- 45} & 270 & {- 135} & 180 & 135 & {- 270} & 45\end{bmatrix}\quad$

In this way, by combining the above two codebooks along the columndirection, a three-bit codebook for eight dual-polarized antennas asshown below may be obtained.

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135} \\0 & 180 & 0 & 180 & 0 & 180 & 0 & 180 \\0 & {- 135} & 90 & {- 45} & 180 & 45 & {- 90} & 135 \\0 & {- 90} & 180 & 90 & 0 & {- 90} & {- 180} & 90 \\0 & {- 45} & 270 & {- 135} & 180 & 135 & {- 270} & 45\end{bmatrix}\quad$

Sixth Embodiment

In the fifth embodiment, both the basic codebook and the generatedcodebook with phase offset are three-bit codebooks without addingspecial bits to feed back a phase offset, thereby obtaining a goodprecision of feedback without increasing the feedback overhead. However,in some cases, people may expect to obtain a more precise feedbackprecision and can accept increasing of the feedback overhead. In such acase, the above DFT based three-bit four-antenna codebook can beextended into a four-bit codebook in which the added one bit is used forthe feedback of a phase offset. The example will be described in thepresent embodiment.

In the present embodiment, a four-bit codebook obtained by extending theabove DFT based three-bit four-antenna codebook, i.e., combining two DFTbased three-bit four-antenna codebook in the row direction, is used forthe first antenna array:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & 0 \\0 & 45 & 90 & 135 & 180 & {{- 135}\mspace{11mu}} & {{- 90}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {0\mspace{11mu}} & {45\mspace{11mu}} & {90\mspace{11mu}} & {135\mspace{11mu}} & {180\mspace{11mu}} & {{{- 13}\; 5}\mspace{11mu}} & {{- 90}\mspace{11mu}} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & {90\mspace{11mu}} & {{- 180}\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {0\mspace{11mu}} & {90\mspace{11mu}} & {{- 180}\mspace{11mu}} & {- 90} \\0 & 135 & 270 & 45 & 180 & {{- 45}\mspace{11mu}} & {{- 270}\mspace{11mu}} & {{- 135}\mspace{11mu}} & {0\mspace{11mu}} & {135\mspace{11mu}} & {270\mspace{11mu}} & {45\mspace{11mu}} & {180\mspace{11mu}} & {{- 45}\mspace{11mu}} & {{- 270}\mspace{11mu}} & {- 135}\end{bmatrix}\quad$

The 8 pre-coding matrix pairs having high correlations are generatedfrom 16 pre-coding matrices in the codebook in a pairing manner of {1,2} {3, 4} {5, 6} {7, 8} {9, 10} {11, 12} {13, 14} {15, 16} (respectivenumbers represent sequence numbers for pre-coding matrices in acodebook), and for each of the pre-coding matrix pairs, a phase offsetis assigned to two pre-coding matrices contained therein according towhether the correlation of the two pre-coding matrices is high or low.Specifically, in the present embodiment, for each of the matrix pairs{1, 2} {3, 4} {5, 6} {7, 8}, a phase offset of 0 is assigned to onepre-coding matrix therein, and a phase offset of 90° is assigned to theother pre-coding matrix; for each of the matrix pairs {9, 10} {11, 12}{13, 14} {15, 16}, a phase offset of 180° is assigned to one pre-codingmatrix therein, and a phase offset of −90° is assigned to the otherpre-coding matrix, the assignment of phase offset is shown as follows:

$\left\lbrack {\begin{matrix}{{0 + 0}\mspace{14mu}} & {{0 + 90}\mspace{14mu}} & {{0 + 0}\mspace{14mu}} & {{0 + 90}\mspace{14mu}} & {{0 + 0}\mspace{14mu}} & {{0 + 90}\mspace{14mu}} & {{0 + 0}\mspace{14mu}} & {0 + 90} \\{{0 + 0}\mspace{14mu}} & {{45 + 90}\mspace{14mu}} & {{90 + 0}\mspace{14mu}} & {{135 + 90}\mspace{14mu}} & {{180 + 0}\mspace{14mu}} & {{{- 135} + 90}\mspace{14mu}} & {{{- 90} + 0}\mspace{14mu}} & {{- 45} + 90} \\{{0 + 0}\mspace{14mu}} & {{90 + 90}\mspace{14mu}} & {{180 + 0}\mspace{14mu}} & {{{- 90} + 90}\mspace{14mu}} & {{0 + 0}\mspace{14mu}} & {{90 + 90}\mspace{14mu}} & {{{- 180} + 0}\mspace{14mu}} & {{- 90} + 90} \\{{0 + 0}\mspace{14mu}} & {{135 + 90}\mspace{14mu}} & {{270 + 0}\mspace{14mu}} & {{45 + 90}\mspace{14mu}} & {{180 + 0}\mspace{14mu}} & {{{- 45} + 90}\mspace{14mu}} & {{{- 270} + 0}\mspace{14mu}} & {{- 135} + 90}\end{matrix}\begin{matrix}{{0 + 180}\mspace{14mu}} & {{0 - 90}\mspace{14mu}} & {{0 + 180}\mspace{14mu}} & {{0 - 90}\mspace{14mu}} & {{0 + 180}\mspace{14mu}} & {{0 - 90}\mspace{14mu}} & {{0 + 180}\mspace{14mu}} & {0 - 90} \\{{0 + 180}\mspace{14mu}} & {{45 - 90}\mspace{14mu}} & {{90 + 180}\mspace{14mu}} & {{135 - 90}\mspace{14mu}} & {{180 + 180}\mspace{14mu}} & {{{- 135} - 90}\mspace{14mu}} & {{{- 90} + 180}\mspace{14mu}} & {{- 45} - 90} \\{{0 + 180}\mspace{14mu}} & {{90 - 90}\mspace{14mu}} & {{180 + 180}\mspace{14mu}} & {{{- 90} - 90}\mspace{14mu}} & {{0 + 180}\mspace{14mu}} & {{90 - 90}\mspace{14mu}} & {{{- 180} + 180}\mspace{14mu}} & {{- 90} - 90} \\{{0 + 180}\mspace{14mu}} & {{135 - 90}\mspace{14mu}} & {{270 + 180}\mspace{14mu}} & {{45 - 90}\mspace{14mu}} & {{180 + 180}\mspace{14mu}} & {{{- 45} - 90}\mspace{14mu}} & {{{- 270} + 180}\mspace{14mu}} & {{- 135} - 90}\end{matrix}} \right\rbrack$

Thereby, a codebook with phase offset is obtained for the second antennaarray:

$\begin{bmatrix}0 & 90 & 0 & 90 & 0 & 90 & {0\mspace{11mu}} & {90\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {{- 90}\mspace{11mu}} & {180\mspace{11mu}} & {- 90} \\0 & 135 & 90 & 225 & 180 & {- 45} & {{- 90}\mspace{11mu}} & {45\mspace{11mu}} & {180\mspace{11mu}} & {{- 45}\mspace{11mu}} & {270\mspace{11mu}} & {45\mspace{11mu}} & {360\mspace{11mu}} & {{- 225}\mspace{11mu}} & {90\mspace{11mu}} & {- 135} \\0 & 180 & 180 & 0 & 0 & 180 & {{- 180}\mspace{11mu}} & {0\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {360\mspace{11mu}} & {{- 180}\mspace{11mu}} & {180\mspace{11mu}} & {0\mspace{11mu}} & {0\mspace{11mu}} & {- 180} \\0 & 225 & 270 & 135 & 180 & 45 & {{- 270}\mspace{11mu}} & {{- 45}\mspace{11mu}} & {180\mspace{11mu}} & {45\mspace{11mu}} & {450\mspace{11mu}} & {{- 45}\mspace{11mu}} & {360\mspace{11mu}} & {{- 135}\mspace{11mu}} & {{- 90}\mspace{11mu}} & {- 225}\end{bmatrix}\quad$

The above pre-coding matrices are equivalent to the followings:

$\begin{bmatrix}0 & 90 & 0 & 90 & {0\mspace{11mu}} & 90 & {0\mspace{11mu}} & 90 & {180\mspace{11mu}} & {- 90} & {180\mspace{11mu}} & {- 90} & {180\mspace{11mu}} & {- 90} & {180\mspace{11mu}} & {- 90} \\0 & 135 & 90 & {- 135} & {180\mspace{11mu}} & {- 45} & {{- 90}\mspace{11mu}} & 45 & {180\mspace{11mu}} & {- 45} & {270\mspace{11mu}} & 45 & {0\mspace{11mu}} & 135 & {90\mspace{11mu}} & {- 135} \\0 & 180 & 180 & 0 & {0\mspace{11mu}} & 180 & {{- 180}\mspace{11mu}} & 0 & {180\mspace{11mu}} & 0 & {0\mspace{11mu}} & {- 180} & {180\mspace{11mu}} & 0 & {0\mspace{11mu}} & {- 180} \\0 & {- 135} & 270 & 135 & {180\mspace{11mu}} & 45 & {{- 270}\mspace{11mu}} & {- 45} & {180\mspace{11mu}} & 45 & {90\mspace{11mu}} & {- 45} & {0\mspace{11mu}} & {- 135} & {{- 90}\mspace{11mu}} & 135\end{bmatrix}\quad$

The same as the fifth embodiment, by combining the above two codebooksalong the column direction, a four-bit codebook for the eightdual-polarized antennas can be obtained as follows.

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} & 0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} & 0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135} & 0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135} \\0 & 90 & 0 & 90 & 0 & 90 & 0 & 90 & 180 & {- 90} & 180 & {- 90} & 180 & {- 90} & 180 & {- 90} \\0 & 135 & 90 & {- 135} & 180 & {- 45} & {- 90} & 45 & 180 & {- 45} & 270 & 45 & 0 & 135 & 90 & {- 135} \\0 & 180 & 180 & 0 & 0 & 180 & {- 180} & 0 & 180 & 0 & 0 & 180 & 180 & 0 & 0 & {- 180} \\0 & {- 135} & 270 & 135 & 180 & 45 & {- 270} & {- 45} & 180 & 45 & 90 & {- 45} & 0 & {- 135} & {- 90} & 135\end{bmatrix}\quad$

Since not only the phase offset of 180° but also the phase offsets of90° and −90° can be fed back by employing the four-bit codebook, theprecision of feedback is further improved.

Seventh Embodiment

In the present embodiment, a four-bit codebook is applied to the eightdual-polarized antennas as shown in FIG. 6, in which a Rel-8four-antenna codebook is applied for the first antenna array, and acodebook with phase offset, which is generated according to the firstembodiment of the present disclosure by using the Rel-8 four-antennacodebook as a basic codebook, is employed for the second antenna array.The Rel-8 four-antenna codebook and the codebook with phase offsetgenerated from the Rel-8 four-antenna codebook have been explained inthe second embodiment of the present disclosure, and unnecessary detailsthereof will not be given herein.

As in the fifth and the sixth embodiments, by combining the Rel-8four-antenna codebook and the generated codebook with phase offset alongthe column direction, a four-bit codebook for eight dual-polarizedantennas can be obtained.

The description of methods of generating codebook with phase offset hasbeen made through multiple embodiments as above. In the following, awireless communication system, base station and terminal using thegenerated codebook with phase offset to communicate will be described.

As described above, multiple codebooks as mentioned above may be appliedin the downlink CoMP environment as shown in FIG. 1. In a wirelesscommunication system including multiple base stations and terminals asshown in FIG. 1, each of the multiple base stations is configured withone antenna array, and the multiple antenna arrays configured in themultiple base stations respectively correspond to multiple codebooks.FIG. 7 and FIG. 8 show structural diagrams of the base stations and theterminals respectively.

As shown in FIG. 7, any base station 700 of the multiple base stationsincludes: a feedback information reception unit 701 which receivesinformation fed back from terminals; a codebook conservation unit 702which forms a codebook with phase offset by assigning phase offsets tocertain pre-coding matrices in a basic codebook, or stores the basiccodebook and the codebook with phase offset formed by assigning phaseoffsets to certain pre-coding matrices in the basic codebook in advance;and a sender unit 703 which selects a corresponding pre-coding matrixfrom the basic codebook or the codebook with phase offset according tothe information fed back from a terminal, uses the selected pre-codingmatrix to pre-code data to be transmitted, and sends the pre-coded datato the terminal via the antenna array configured in the base station.The codebook conservation unit 702 may further includes: a correlationdetermination unit 7021 which determines the pre-coding matrix pairshaving high correlations in the basic codebook; and a phase offsetassignment unit 7022 which assigns a phase offset to two pre-codingmatrices included in a pre-coding matrix pair according to thecorrelation between the two pre-coding matrices, such that the higherthe correlation between the two pre-coding matrices is, the larger theassigned phase offset is.

It should be explained that although it is described here that thesender unit 703 selects a corresponding pre-coding matrix from the basiccodebook or the codebook with phase offset according to the informationfed back from a terminal, it is up to the base station if the actualdownlink data transmission would follow terminal's suggestion.Specifically, the sender unit 703 may consider not only the feedbackinformation, but also other factors well known for those skilled in theart such as feedback information from other terminals, backgroundconnection quality between the base stations, the degree of sharingschedule information between the base stations, and the like, to selecta proper pre-coding matrix from the codebook. Therefore, the sender unit703 is not limited to select the suggested pre-coding matrix from thebasic codebook or the codebook with phase offset, instead, it may selectother proper pre-coding matrices for downlink data transmissionaccording to other factors well known for those skilled in the art.

As shown in FIG. 8, a terminal 800 includes: a codebook conservationunit 801 which forms a codebook with phase offset by assigning phaseoffsets to certain pre-coding matrices in a basic codebook, or storesthe basic codebook and the codebook with phase offset formed byassigning phase offsets to certain pre-coding matrices in the basiccodebook in advance; and a information feedback unit 802 which feedsback indexes for identifying pre-coding matrices in the basic codebookand the codebook with phase offset to respective antenna arrays of thebase stations. The codebook conservation unit 801 may further include: acorrelation determination unit 8011 which determines the pre-codingmatrix pairs having high correlations in the basic codebook; and a phaseoffset assignment unit 8012 which assigns a phase offset to twopre-coding matrices included in a pre-coding matrix pair according tothe correlation between the two pre-coding matrices, such that thehigher the correlation between the two pre-coding matrices is, thelarger the assigned phase offset is.

It is easy to understand that in a wireless communication systemincluding the above base stations and terminals described with referenceto FIG. 7 and FIG. 8, the proper codebooks may be applied for respectivebase stations according to the manners described in the second, thirdand fourth embodiments as above. Accordingly, a terminal will feed backindexes for identifying the pre-coding matrices according to thecodebooks used by respective base stations.

The present disclosure may also be applied to the eight dual-polarizedantennas as shown in FIG. 6. In a wireless communication systemincluding the base stations and terminals configured with dual-polarizedantennas, the base stations have structures similar with that as shownin FIG. 7. Specifically, the base station is configured with two antennaarrays formed as the dual-polarized antennas, and includes: a feedbackinformation reception unit for receiving information fed back fromterminals; a codebook conservation unit for forming a codebook withphase offset by assigning phase offsets to certain pre-coding matricesin a basic codebook, or for storing a codebook obtained by combining thebasic codebook and the codebook with phase offset generated from thebasic codebook in advance, in which the two antenna arrays in thedual-polarized antennas correspond to the basic codebook and thecodebook with phase offset respectively; and a sender unit for selectinga corresponding pre-coding matrix from the codebook obtained bycombining the basic codebook and the codebook with phase offsetaccording to the information fed back from a terminal, using theselected pre-coding matrix to pre-code data to be transmitted, andsending the pre-coded data to the terminal via the antenna arrays. Thecodebook conservation unit may further include: a correlationdetermination unit for determining pre-coding matrix pairs having highcorrelations in the basic codebook; and a phase offset assignment unitfor assigning a phase offset to two pre-coding matrices according to thecorrelation between the two pre-coding matrices, such that the higherthe correlation between the two pre-coding matrices is, the larger theassigned phase offset is.

Also, although it is described herein that the sender unit selects acorresponding pre-coding matrix from the codebook obtained by combiningthe basic codebook and the codebook with phase offset according to theinformation fed back from a terminal, this is not limiting, instead, aproper pre-coding matrix can be selected according to other factors wellknown for those skilled in the art.

In the above wireless communication system including base stations andterminals configured with dual-polarized antennas, the structures of theterminals is the same as the structure shown in FIG. 8, thus unnecessarydetails thereof will not be given herein. It should be explained that,as mentioned in the fifth embodiment, the pre-coding matrix indexes forthe two antenna arrays in the dual-polarized antennas are highlycorrelative, thus the pre-coding matrix index for the second antennaarray is always the same as the pre-coding matrix index for the firstantenna array. In such a case, the information feedback unit of theterminal may feed back only the index for the pre-coding matrixsuggested to be used by the first antenna array, or only the index forthe pre-coding matrix suggested to be employed by the second antennaarray.

It is easy to understand that in a wireless communication systemincluding base stations and terminals configured with dual-polarizedantennas, the proper codebooks may be applied for antenna arrays in thebase stations according to the manners described in the fifth, sixth andseventh embodiments as above. Accordingly, a terminal may feed backindexes for identifying the pre-coding matrices according to codebooksused by the antenna arrays in the base stations.

Eighth Embodiment

The codebook designs according to the foregoing embodiments efficientlyimprove the precision of feedback for a multi-antenna array, thusbringing a good throughput for a single user. In the present embodiment,a further description will be given about how to improve the datathroughput in case of multiple user transmission, provided that theperformance of single user is not decreased.

FIG. 9 shows an exemplary systematical architecture of a multi-usertransmission system.

FIG. 9 is a case in which a base station serves multiple clients at thesame time. It should be explained herein that the base station in FIG. 9is described as having a form of eight dual-polarized antennas, it isonly exemplary, and the base station of the multi-user transmissionsystem may be any other forms well known in the art. When differentreporting manners are employed in the clients, the operation proceduresof the multi-user transmission system are different either. Theoperation procedures of the multi-user transmission system in two casesare introduced below.

First of all, when the clients employ a reporting manner of PMI, theoperation procedure of the system is usually as follows. First, theclients report their own PMIs to the base station (the clients assumethat the base station employs single-user transmission). Then, the basestation detects the PMIs of the multiple clients, and sends data of twoclients on the same resource block if the PMIs reported by the twoclients are orthogonal with each other, otherwise, the base station willnot perform multi-user data transmission. Finally, the clients receivedata from the base station and demodulate the received data withoutknowing whether there is a co-scheduled UE or not.

Further, in addition to the above reporting manner of PMI, the currentstandard may also employ a reporting manner of PMI+BCI (Best CompanionPMI). The operation procedure of the system in such a case is asfollows. First, in addition to the clients' own PMIs (clients assumethat the base station employs single-user transmission), the clientsfurther report the PMIs expected most to be used by a co-scheduled UE ina case of utilizing multi-user transmission, i.e., BCIs (in principle,the BCI should use the same codebook as the PMI), to the base station.Then, the base station detects the PMIs and BCIs of the multipleclients, and sends data of two clients on the same resource block if theBCI reported by one client is the same as the PMI reported by the otherclient, otherwise, the base station will not perform multi-user datatransmission. Finally, the clients receive data from the base stationand demodulate the received data without knowing whether there is aco-scheduled UE or not.

It can be seen, from the operation procedures of the multi-usertransmission system in the above two cases, that the possibility ofmulti-user transmission is largely dependent on the orthogonal propertybetween PMIs (or BCIs) reported by the multiple clients. In general, itis possible to perform multi-user transmission for multiple users onlyin a case that the PMIs (or BCIs) reported by the multiple clients areorthogonal with each other. Therefore, in order to improve performanceof multi-user transmission, the orthogonal property between thepre-coding vectors (corresponding to the pre-coding matrices asmentioned in the above embodiments) included in a codebook should besufficiently considered when a codebook is designed.

In the following, the orthogonal property between pre-coding vectors ina codebook is specifically analyzed by taking the eight dual-polarizedantennas of the fifth embodiment and the sixth embodiment as an example.

First, for convenience of orthogonal property analysis, the pre-codingmatrices in the DFT based three-bit four-antenna codebook represented inphase in the above fifth embodiment

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135}\end{bmatrix}\quad$

are represented in a order of from left to right respectively as basevectors v0, v1, v2, v3, v4, v5, v6, v7, that is:

v0=[0 0 0 0]^(T)

v1=[0 45 90 135]^(T)

v2=[0 90 180 270]^(T)

v3=[0 135 −90 45]^(T)

v4=[0 180 0 180]^(T)

v5=[0 −135 90 −45]T

v6=[0 −90 −180 −270]^(T)

v7=[0 −45 −90 −135]^(T)

FIG. 10 shows the grouped orthogonal property of base vectors in a DFTbased three-bit four-antenna codebook. As shown in FIG. 10, therespective base vectors as described above are grouped orthogonally,that is, the base vectors v0, v2, v4, v6 constitute an orthogonal basis,and the base vectors v1, v3, v5, v7 constitute another orthogonal basis.It should be noted that the mapping relationship between the respectivebase vectors v0, v1, v2, v3, v4, v5, v6, v7 and the vector graph of FIG.10 is well known for those skilled in the art, and it will not bedescribed in detail herein.

According to the above sixth embodiment, by combining the above DFTbased three-bit four-antenna codebook in the row direction into afour-bit codebook, and by combining the obtained four-bit codebook and acodebook with phase offset generated from the obtained four-bit codebookin the column direction, a four-bit codebook for the eightdual-polarized antennas is obtained as follows:

$\begin{bmatrix}0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} & 0 & 45 & 90 & 135 & 180 & {- 135} & {- 90} & {- 45} \\0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} & 0 & 90 & 180 & {- 90} & 0 & 90 & {- 180} & {- 90} \\0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135} & 0 & 135 & 270 & 45 & 180 & {- 45} & {- 270} & {- 135} \\0 & 90 & 0 & 90 & 0 & 90 & 0 & 90 & 180 & {- 90} & 180 & {- 90} & 180 & {- 90} & 180 & {- 90} \\0 & 135 & 90 & {- 135} & 180 & {- 45} & {- 90} & 45 & 180 & {- 45} & 270 & 45 & 0 & 135 & 90 & {- 135} \\0 & 180 & 180 & 0 & 0 & 180 & {- 180} & 0 & 180 & 0 & 0 & 180 & 180 & 0 & 0 & {- 180} \\0 & {- 135} & 270 & 135 & 180 & 45 & {- 270} & {- 45} & 180 & 45 & 90 & {- 45} & 0 & {- 135} & {- 90} & 135\end{bmatrix}\quad$

Here, each column in the four-bit codebook for eight dual-polarizedantennas is referred to as a pre-coding vector (pre-coding matrix).According to the above definitions for the respective base vectors v0,v1, v2, v3, v4, v5, v6, v7, every pre-coding vector may be representedby the above base vectors. The base vectors in the basic codebook mayalso be referred to as the pre-coding vectors. Relationships between theindexes for pre-coding matrix and the pre-coding vectors represented bythe above base vectors are shown in Table 1 below.

TABLE 1 Pre-coding Pre-coding matrix indexes vectors PMI0 [v0 v0]^(T)PMI1 [v1 jv1]^(T) PMI2 [v2 v2]^(T) PMI3 [v3 jv3]^(T) PMI4 [v4 v4]^(T)PMI5 [v5 jv5]^(T) PMI6 [v6 v6]^(T) PMI7 [v7 jv7]^(T) PMI8 [v0 −v0]^(T)PMI9 [v1 −jv1]^(T) PMI10 [v2 −v2]^(T) PMI11 [v3 −jv3]^(T) PMI12 [v4−v4]^(T) PMI13 [v5 −jv5]^(T) PMI14 [v6 −v6]^(T) PMI15 [v7 −jv7]^(T)

The pre-coding matrix indexes PMI0, PMI1, . . . , PMI5 correspond toevery column from left to right in the above four-bit codebook for theeight dual-polarized antennas respectively, that is, every pre-codingvectors from left to right. Coefficients 1, j, −1, −j correspond to thephase offset assignments of 0°, 90°, 180°, −90° respectively. Withrespect to the orthogonal property between the pre-coding vectors in theabove four-bit codebook for the eight dual-polarized antennas, we takethe pre-coding vector corresponding to the PMI0 in Table 1 as an exampleto analyze the orthogonal property.

FIG. 11 shows the orthogonal property of the pre-coding vectorcorresponding to the PMI0 in the four-bit codebook for eightdual-polarized antennas according to the sixth embodiment.

It can be seen from FIG. 11 that the pre-coding vector corresponding tothe PMI0 is orthogonal with the pre-coding vectors corresponding toseven PMIs in total of PMI2, PMI4, PMI6, PMI5, PMI10, PMI12 and PMI14respectively. This is because, as describe above, the base vectors v0,v2, v4, v6 constitute a set of orthogonal bases with each other, thepre-coding vectors based on the base vectors v0, v2, v4, v6 areorthogonal with each other. For the same reason, eight pre-codingvectors based on the base vectors v1, v3, v5, v7, that is, pre-codingvectors corresponding to PMI1, PMI3, PMI5, PMI7, PMI9, PMI11, PMI13,PMI15 respectively, are orthogonal with each other. Therefore, in a casethat the original codebook is used, the user using the base vector“v0(PMI0, PMI8)” is able to perform the multi-user transmission withonly the users having the base vector “v2, v4 or v6(PMI2, PMI4, PMI6,PMI10, PMI12, PMI14)”.

In addition, it can be noted from FIG. 10 that the angle differencesbetween v0 and v2, v4, v6 are 90°, 180°, −90° respectively, while theangle differences between v0 and v1, v7 are 45°, −45° respectively. Thismeans that the codebook design limits the scheduler of the base station,that is, it makes the scheduler perform the multi-user transmission withrespect to only the multiple users with angle differences of a multipleof 90°, but not for the multiple users with angle differences of 45°.

Therefore, in order to improve the performance of multi-usertransmission, the orthogonal property between the pre-coding vectors inthe codebook should be further improved when the codebook is designed.

However, it should be noted herein that in the progress of the existingstandardization, the single-user performance is generally of higherpriority than the multi-user performance. Thus, the codebook design withrespect to the multiple users should be made on the premise that thesingle-user performance is not degraded. Also, the single-userperformance is mainly dependent on two aspects: (1) directionalities ofthe base vectors (eight vectors distributed uniformly in space); and (2)the precision of feedback for phase offset (two bits). Therefore, thecodebook improvement for the multiple users should be made on thepremise that the above two aspects are not influenced.

In the present embodiment, a new codebook is generated by designing aset of new base vectors v′0, v′1, v′2, v′3, v′4, v′5, v′6, v′7satisfying the following attributes to replace v0, v1, v2, v3, v4, v5,v6, v7 respectively in the pre-coding vectors corresponding to the PMI0to the PMI7, while keeping the pre-coding vectors corresponding to thePMI8 to the PMI15 unchanged:

(1) The directionalities of the major lobes of v′0, v′1, v′2, v′3, v′4,v′5, v′6, v′7 are the same as those of v0, v1, v2, v3, v4, v5, v6, v7respectively;

(2) v′0, v′2, v′4, v′6 constitute a set of orthogonal bases of fourdimensions, and v′1, v′3, v′5, v′7 constitute another set of orthogonalbases of four dimensions;

(3) v′0, v′2, v′4, v′6 are orthogonal with at least one of v1, v3, v5,v7 respectively, and v′1, v′3, v′5, v′7 are orthogonal with at least oneof v0, v2, v4, v6 respectively.

The new codebook thus generated can improve the performance of themulti-user transmission. Now, the pre-coding vector corresponding to thePMI0 is still taken as an example, and the orthogonal property after theabove improvement is analyzed.

FIG. 12 shows the orthogonal property of the pre-coding vectorcorresponding to the PMI0 in the four-bit codebook for eightdual-polarized antennas according to the present embodiment.

It can be seen from FIG. 12 that the improved pre-coding vector [v′0v′0]^(T) corresponding to the PMI0 are orthogonal with, in addition tothe pre-coding vectors corresponding to the seven PMIs of PMI2, PMI4,PMI6, PMI5, PMI10, PMI12, PMI14 respectively, the pre-coding vectorscorresponding to the PMI9 and PMI15. Thus, the scheduler of the basestation is enabled to perform the multi-user transmission for not onlythe multiple users with angle differences of a multiple of 90° but alsothe multiple users with angle differences of 45°.

It should be explained that the example given by FIG. 12 is a case ofv′0 being orthogonal with v1 and v7, which is only exemplary. Theorthogonal property of the pre-coding vector corresponding to the PMI0can be improved as long as v′0 is orthogonal with any one of v1, v3, v5,v7. The more vectors of v1, v3, v5, v7 with which v′0 is orthogonal,more highly the orthogonal property of the pre-coding vectorcorresponding to the PMI0 can be improved, that is, more highly theperformance of the multi-user transmission can be improved.

The above new codebook not only improves the performance of themulti-user transmission, but also does not reduce the single-userperformance. This is because the directionalities of newly designed basevectors v′0, v′1, v′2, v′3, v′4, v′5, v′6, v′7 are made to be consistentwith those of v0, v1, v2, v3, v4, v5, v6, v7 respectively, that is, theyare still eight vectors distributed uniformly in space, by satisfyingthe attribute (1). And, the phase offset assigned in the sixthembodiment is not changed, that is, the precision of phase offset isstill two bits. Accordingly, the above two aspects which decide thesingle-user performance are both satisfied. Thus, the new codebookobtained in the present embodiment can improve the performance of themulti-user transmission on the premise that the single-user performanceis not reduced.

More specifically, as an example, the present disclosure provides amethod by which the above new base vectors v′0, v′1, v′2, v′3, v′4, v′5,v′6, v′7 can be efficiently obtained as follows:

At a first step, v′0 is generated, that is, a vector that can maximizethe correlation with v0 is found among the vectors being orthogonal withat least one of v1, v3, v5, v7, and it is taken as v′0.

It can be known from the above description that there are two sets ofbase vectors (pre-coding matrices) such as the base vectors v0, v2, v4,v6 and the base vectors v1, v3, v5, v7 included in the basic codebook,and respective base vectors in each set of base vectors are orthogonalwith each other. According to the present disclosure, it is to find out(compute) vectors that are orthogonal with at least one base vector in aset of base vectors v1, v3, v5, v7 (considered as the second set of basevectors here), and a vector whose correlation with one base vector (forexample v0) of another set of base vectors v0, v2, v4, v6 (considered asthe first set of base vectors here) is the highest among the acquiredvectors is made to be a new base vector, such as v′0. According to thepresent embodiment, the acquirement of the vectors that are orthogonalwith at least one pre-coding matrix of the second set of base vectorscan be realized by computing a linear combination of at least two basevectors of the second set of base vectors, for example, computing thelinear combination of v3 and v5.

At a second step, other new base vectors v′1, v′2, v′3, v′4, v′5, v′6,v′7 other than v′0 are generated one by one through the same operationas the first step. For example, if v′1 is to be generated, it isrequired to find out a vector that can maximize the correlation with v1among the vectors being orthogonal with at least one of v2, v3, v6, v8,and take it as v′1. With respect to the other new base vectors v′2, v′3,v′4, v′5, v′6, v′7, the same operation is employed.

Additionally, after any one of v′0, v′1, v′2, v′3, v′4, v′5, v′6, v′7 isgenerated, the other new base vectors can be generated with a manner asfollow. For example, after v′0 is generated, the other new base vectorsv′1, v′2, v′3, v′4, v′5, v′6, v′7 can be obtained by the equation below,for example:

v′1 can be obtained via v′1=diag (v1)*v′0, i.e.,

${v^{\prime}1} = {{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & \frac{1 + j}{\sqrt{2}} & 0 & 0 \\0 & 0 & j & 0 \\0 & 0 & 0 & \frac{{- 1} + j}{\sqrt{2}}\end{bmatrix} \cdot v^{\prime}}0}$

Similarly, v′2, v′3, v′4, v′5, v′6, v′7 can be obtained respectivelythrough the following equations:

v^(′)2 = diag (v 2) * v^(′)0 … v^(′)7 = diag (v 7) * v^(′)0

That is to say, the other base vectors v1, v2, v3, v4, v5, v6, v7 otherthan the base vector v0 are further multiplied with the generated newbase vector v′0 respectively, such that the other new base vectors v′1,v′2, v′3, v′4, v′5, v′6, v′7 are generated.

Subsequently, the original base vectors v0, v1, v2, v3, v4, v5, v6, v7are respectively replaced with the new base vectors v′0, v′1, v′2, v′3,v′4, v′5, v′6, v′7, thus a new basic codebook is formed.

According to the present embodiment, the basic codebook [v0, v1, v2, v3,v4, v5, v6, v7] and the new codebook [v′0, v′1, v′2, v′3, v′4, v′5, v′6,v′7] are combined along the row direction to form an extended codebook,and a predetermined base vectors in the extended codebook are assignedphase offsets so that a codebook with phase offset is generated.Further, the extended codebook and the generated codebook with phaseoffsets are combined along the column direction into one codebook as thecodebook for the eight dual-polarized antennas.

For further convenience of understanding, the design for v′0 in thefirst step is now illustrated. Here, it is assumed that v0=[1 1 11]^(T), and it is to find out a vector which can maximize thecorrelation with [1 1 1 1]^(T) among the vectors being orthogonal withv1 and v7, and take it as v′0. According to the present embodiment, thevectors being orthogonal with v1 and v7 may be linear combinations of v3and v5. By performing a numerical value search, the following equationcan be obtained.

${v^{\prime}0} = {{{v\; {3/\sqrt{2}}} + {\left( {0.5 + {0.5j}} \right)*v\; 5}} = \begin{bmatrix}{1.2071 + {0.5000i}} \\{{- 0.5000} - {0.2071i}} \\{{- 0.5000} - {0.2071i}} \\{1.2071 + {0.5000i}}\end{bmatrix}}$

Ninth Embodiment

The codebook design in the eighth embodiment makes the scheduler of abase station be able to perform multi-user transmission for multipleusers with angle differences of not only multiples of 90° but also 45°,thereby it improves the performance of multi-user transmission on thepremise that the single-user performance is not degraded.

The same manner as that in the eighth embodiment above may be extendedto a case in that the rank of a pre-coding matrix is 2, as described inthe ninth embodiment.

The case of the rank 2 transmission has a similar nature with MU-MIMOtransmission, i.e., signals in more than one layer are sent out from abase station, with the multi-user transmission in the eighth embodiment.Their difference resides in that the signals in two layers are sent totwo users respectively in the multi-user transmission, while the signalsin the two layers are sent to the same user in the case of the rank 2transmission. Similar with the eighth embodiment, the emissiondirections of the signals in the two layers are v0 and v′0 respectively.

A basic form of a rank 2 pre-coding matrix for eight dual-polarizedantennas is

$\begin{bmatrix}v_{0} & v_{0} \\v_{0} & {- v_{0}}\end{bmatrix},$

that is, the pre-coding matrix vector used by the signals in the firstlayer is

$\begin{bmatrix}v_{0} \\v_{0}\end{bmatrix},$

and the pre-coding matrix vector used by the signals in the second layeris

$\begin{bmatrix}v_{0} \\{- v_{0}}\end{bmatrix}.$

Wherein, v₀ is a DFT vector with the length of 4, and it corresponds toa angle of θ. The basic form represents that the transmission directionsof the signals in the first layer and the second layer are identical,i.e., v0=v′0, and they have completely opposite phase offset. Theopposite phase offset are mainly used to assure the orthogonal propertybetween the precoding vectors of the two layers, which is an essentialfeature of rank 2 pre-coding matrix.

It is noted that in a practical environment, the best directions of thesignals in the first layer and the second layer are not necessarily tobe exactly the same, but they are possibly different. Therefore, ifcodebook includes certain pre-coding matrixes corresponding to the casethat the directions of the signals in the first layer and the secondlayer are different, the throughput of the system can be improved.

In the eighth embodiment, the first step is as follows:

(1) the directions of major lobes of v′0, v′1, v′2, v′3, v′4, v′5, v′6,v′7 are the same as those of v0, v1, v2, v3, v4, v5, v6, v7respectively;

The v′0, v′1, v′2, v′3, v′4, v′5, v′6, v′7 in the eighth embodimentrepresent pre-coding vectors used for the second user in MU-MIMOtransmission, and they represent pre-coding vectors used by the signalsin the second layer for the rank 2 case. In the basic form of rank 2,the directions of signals in the second layer have already been the sameas those of signals in the first layer. The improvement of the presentembodiment is to expect the directions of signals in the second layerand the first layer to be different, thus the first step of the ninthembodiment should be as follows:

(1) the directions of major lobes of v′0, v′1, v′2, v′3, v′4, v′5, v′6,v′7 are different with those of v0, v1, v2, v3, v4, v5, v6, v7respectively;

The second step of the ninth embodiment is the same as that of theeighth embodiment:

(2) v′0, v′2, v′4, v′6 constitute a set of orthogonal bases in fourdimensions, and v′1, v′3, v′5, v′7 constitute another set of orthogonalbases in four dimensions;

The third step of the ninth embodiment is also the same as that of theeighth embodiment:

(3) v′0, v′2, v′4, v′6 are orthogonal with at least one of v1, v3, v5,v7 respectively, and v′1, v′3, v′5, v′7 are orthogonal with at least oneof v0, v2, v4, v6 respectively.

One special form of the third step is that v′0, v′2, v′4, v′6 areorthogonal with three of v1, v3, v5, v7 respectively. For instance, v′0is orthogonal with v3, v5, v7, so that v′0=v1, similarly, v′2=v3, v′4=v5and v′6=v7. In addition, v′1, v′3, v′5, v′7 are orthogonal with three ofv0, v2, v4, v6 respectively, so that it can be obtained that v′1=v2,v′3=v4, v′5=v6 and v′7=v0. For convenience of reading, we summarize therelationship between v′ and v as follows:

v′0=v1

v′1=v2

v′2=v3

v′3=v4

v′4=v5

v′5=v6

v′6=v7

v′7=v0

Thereby, we can deduce a rank 2 pre-coding matrix in accordance with theprinciple of the eighth embodiment. It is assumed that the basicpre-coding matrixes are in following forms:

${{PMI}\; 0{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 0} & {v\; 0} \\{v\; 0} & {{- v}\; 0}\end{bmatrix}}},{{PMI}\; 1{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 0} & {v\; 0} \\{j\; v\; 0} & {{- j}\; v\; 0}\end{bmatrix}}},{{PMI}\; 2{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 2} & {v\; 2} \\{v\; 2} & {{- v}\; 2}\end{bmatrix}}},{{PMI}\; 3{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 2} & {v\; 2} \\{j\; v\; 2} & {{- j}\; v\; 2}\end{bmatrix}}},{{PMI}\; 4{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 4} & {v\; 4} \\{v\; 4} & {{- v}\; 4}\end{bmatrix}}},{{PMI}\; 5{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 4} & {v\; 4} \\{j\; v\; 4} & {{- j}\; v\; 4}\end{bmatrix}}},{{PMI}\; 6{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 6} & {v\; 6} \\{v\; 6} & {{- v}\; 6}\end{bmatrix}}},{{PMI}\; 7{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 6} & {v\; 6} \\{j\; v\; 6} & {{- j}\; v\; 6}\end{bmatrix}}},$

In such a case, according to the present disclosure, another set ofpre-coding matrixes as shown in the following PMI8 to PMI15 can bedesigned based on respective pre-coding matrixes corresponding to PMI0to PMI7 as described above.

${{PMI}\; 8{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 0} & {v\; 1} \\{v\; 0} & {{- v}\; 1}\end{bmatrix}}},{{PMI}\; 9{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 0} & {v\; 1} \\{j\; v\; 0} & {{- j}\; v\; 1}\end{bmatrix}}},{{PMI}\; 10{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 2} & {v\; 3} \\{v\; 2} & {{- v}\; 3}\end{bmatrix}}},{{PMI}\; 11{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 2} & {v\; 3} \\{j\; v\; 2} & {{- j}\; v\; 3}\end{bmatrix}}},{{PMI}\; 12{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 4} & {v\; 5} \\{v\; 4} & {{- v}\; 5}\end{bmatrix}}},{{PMI}\; 13{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 4} & {v\; 5} \\{j\; v\; 4} & {{- j}\; v\; 5}\end{bmatrix}}},{{PMI}\; 14{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 6} & {v\; 7} \\{v\; 6} & {{- v}\; 7}\end{bmatrix}}},{{PMI}\; 15{{\text{:}\mspace{14mu}\begin{bmatrix}{v\; 6} & {v\; 7} \\{j\; v\; 6} & {{- j}\; v\; 7}\end{bmatrix}}.}}$

In the above eight pre-coding matrixes, the direction difference betweenthe signals in the first layer and the signals in the second layer is45°, where v0˜v7 is defined as in the eighth embodiment.

In some cases, for rank one transmission, an angle difference of 45°between the basic pre-coding matrixes is too coarse, and a finer angledifference is preferred. For example, 22.5° or even 11.25° difference indirection is preferred. In the following text, we extend the abovemethod to 11.25° direction difference.

The base vector in case of 11.25° difference can be written as:

w1=[0 11.25 22.5 33.75]

w2=[0 22.5 45 67.5]

w3=[0 33.75 67.5 101.25]

w4=[0 45 90 135]

w5=[0 56.25 112.5 168.75]

w6=[0 67.5 135 202.5]

w7=[0 78.75 157.5 236.25]

w8=[0 90 180 270]

w9=[0 101.25 202.5 303.75]

w10=[0 112.5 225 337.5]

w11=[0 123.75 247.5 11.25]

w12=[0 135 270 45]

w13=[0 146.25 292.5 78.75]

w14=[0 157.5 315 112.5]

w15=[0 168.75 337.5 146.25]

w16=[0 180 0 180]

w17=[0 191.25 22.5 213.75]

w18=[0 202.5 45 247.5]

w19=[0 213.75 67.5 281.25]

w20=[0 225 90 315]

w21=[0 236.25 112.5 348.75]

w22=[0 247.5 135 22.5]

w23=[0 258.75 157.5 56.25]

w24=[0 270 180 90]

w25=[0 281.25 202.5 123.75]

w26=[0 292.5 225 157.5]

w27=[0 303.75 247.5 191.25]

w28=[0 315 270 225]

w29=[0 326.25 292.5 258.75]

w30=[0 337.5 315 292.5]

w31=[0 348.75 337.5 326.25]

To emphasize that the new base vectors has better spatial granularity,we use “w” instead of “v” to represent those base vectors.

The above 32 base vectors include eight sets of orthogonal bases, inwhich the first set of orthogonal bases is [w0 w8 w16 w24], the secondset of orthogonal bases is [w1 w9 w17 w25], the third set of orthogonalbases is [w2 w10 w18 w26], the fourth set of orthogonal bases is [w3 w11w18 w27], and so on, thus all eight sets of orthogonal bases can beobtained.

As described in the above method, the form of the rank 2 pre-codingmatrix to be designed is

$\begin{bmatrix}w_{0} & w_{0}^{\prime} \\w_{0} & {- w_{0}^{\prime}}\end{bmatrix}.$

According to the above steps, the first step of obtaining the rank 2pre-coding matrix is that the direction of w′0 is different with that ofw0, the second step is that [w′0, w′8, w′16, w′24] is one set oforthogonal bases, and the third step is that w′0 is orthogonal withthree of w1˜w31.

One example of the third step is that w′0 is orthogonal with w9, w17,w25. In this case, w′0=w1, and similarly it can be obtained w′1=w2, . .. , w′30=w31 and w′31=w0. In this case, the direction difference betweenthe signals in the first layer and the signals in the second layer is11.25°.

Another example of the third step is that w′0 is orthogonal with w10,w18, w26. In this case, w′0=w2, and similarly it can be obtained w′1=w3,. . . , w′29=w31, w′30=w0 and w′31=w1. In this case, the directiondifference between the signals in the first layer and the signals in thesecond layer is 22.5°.

A third example of the third step is that w′0 is orthogonal with w11,w19, w27. In this case, w′0=w3, and similarly it can be obtained w′1=w4,. . . , w′28=w31, w′29=w0, w′30=w1 and w′31=w2. In this case, thedirection difference between the signals in the first layer and thesignals in the second layer is 33.75°.

An actual codebook may be a combination of the three examples of thethird step as described above, that is, the direction differencesbetween the signals in two layers for some rank 2 pre-coding matrixesare 11.25°, the direction differences for other some rank 2 pre-codingmatrixes are 22.5°, and the direction differences for further some rank2 pre-coding matrixes are 33.75°, and so on.

It can be summarized that the method of generating a codebook in awireless communication system according to the ninth embodiment of thepresent disclosure may include: a setting step of setting a basiccodebook including multiple base vectors grouped into multiple sets ineach of which respective base vectors are orthogonal with each other; acomputation step of computing a new base vector corresponding to onebase vector in one set of the multiple sets of base vectors so that thenew base vector equals to another base vector in other sets of basevectors other than the one set of base vectors, and repeating thiscomputation step to acquire multiple new base vectors corresponding tothe multiple base vectors; and a generation step of generating a newcodebook using the respective new base vectors as described above.

In the above method, the plurality of base vectors in the basic codebookare w_(m), which are DFT vectors, and differences between directions ofbase vectors w_(m) and w_(m+n) are n times of 11.25° in turn, where mand n are integer numbers, and they are also possible to be realnumbers.

In the above method, the obtained new base vectors are w′_(m). w′_(m) isalso DFT vectors but the directions of which are different from thecorresponding base vectors w_(m). Wherein w′_(m)=w_(m+n), for example,it is possible to have w′_(m)=w_(m+1), or w′_(m)=w_(m+2), orw′_(m)=w_(m+3), etc.

In the above method, the form of a rank 2 pre-coding vector in the newbasic codebook is

${\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}\mspace{14mu} {and}\text{/}{{or}\mspace{14mu}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{j\; w_{m}} & {{- j}\; w_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from a basestation respectively, and

$\begin{bmatrix}w_{m} \\{j\; w_{m}}\end{bmatrix}\mspace{14mu} {{and}\mspace{11mu}\begin{bmatrix}w_{m}^{\prime} \\{{- j}\; w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from the basestation respectively.

With the above embodiment of the present disclosure, the abovepre-coding matrixes can be well adapted to realize the pre-coding withhigh performance, even if the directions of signals in the two layersemitted from a base station to a mobile terminal are different.

In the base station 700 and the terminal 800 as shown in FIG. 7 and FIG.8, the method of generating a new codebook by newly designing basevectors to improve the orthogonal property between the pre-codingvectors in a codebook in the eighth and ninth embodiments can also beapplied. For example, the codebook conservation units 702 and 801 in thebase station 700 and the terminal 800 can newly design the base vectors.

For example, in the base station 700 according to the eighth embodiment,a basic codebook includes two sets of base vectors, and respective basevectors in each set of base vectors are orthogonal with each other. Thecodebook conservation unit 702 of the base station 700 acquires thevectors that are orthogonal with at least one base vector of the secondset of base vectors, among the acquired vectors computes a vector whosecorrelation with one base vector of the first set of base vectors is thehighest as a new base vector, generates other new base vectorssimilarly, and replaces respective original base vectors with respectivenew base vectors to form a new basic codebook.

According to the present embodiment, the codebook conservation unit 702of the base station 700 generates other new base vectors from the otherbase vectors except for the one base vector in the first set of basevectors by repeating the operation of generating the new one basevector, or by multiplying the other base vectors except for the one basevector in the first set of base vectors with the new one base vectorrespectively.

According to the present embodiment, the codebook conservation unit 702of the base station 700 computes linear combinations of at least twobase vectors in the second set of base vectors as the vectors beingorthogonal with at least one base vector of the second set of basevectors.

According to the present embodiment, the codebook conservation unit 702of the base station 700 combines the original basic codebook and the newbasic codebook in the row direction to form an extended codebook. Thebase station 700 further assigns the phase offsets to the base vectorsin the extended codebook via the phase offset assignment unit 7022 so asto form a codebook with phase offset.

In a case of multiple antenna arrays applied to eight dual-polarizedantennas (each of antenna arrays includes four antennas having the samepolarization feature of the eight dual-polarized antennas), the codebookconservation unit 702 combines the extended codebook and the generatedcodebook with phase offset along the column direction into one codebookas the codebook for the eight dual-polarized antennas.

For example, in the base station 700 according to the ninth embodiment,a feedback information reception unit 701 receives information fed backfrom a terminal; a codebook conservation unit 702 stores a basiccodebook and/or a new basic codebook, the basic codebook comprising aplurality of base vectors which are grouped into a plurality of sets,respective base vectors in each set of base vectors being orthogonalwith each other, the new basic codebook being obtained in a way of:computating a new base vector corresponding to one base vector in oneset of the plurality of sets of base vectors, so that the new basevector equals to another base vector in other sets of base vectors otherthan the one set of base vectors, and repeating the above operation toacquire a plurality of new base vectors corresponding to the pluralityof base vectors, and generating the new basic codebook using therespective new base vectors; and a sender unit 703 pre-codes data to betransmitted by using the basic codebook and/or the new basic codebookbased on the information fed back from the terminal, and sending thepre-coded data to the terminal via an antenna array.

According to the present embodiment, the plurality of base vectors inthe basic codebook are w_(m), which are DFT vectors, and differencesbetween directions of base vectors w_(m) and w_(m+n) are n times of11.25° in turn, where m and n are integer numbers, and they are alsopossible to be real numbers.

According to the present embodiment, the obtained new base vectors arew′_(m). w′_(m) is also DFT vectors, but the directions of which aredifferent from the corresponding base vectors w_(m). Whereinw′_(m)=w_(m+n), for example, it is possible to have w′_(m)=w_(m+1), orw′_(m)=w_(m+2), or w′_(m)=w_(m+3), etc.

According to the present embodiment, the form of a rank 2 pre-codingvector in the new basic codebook is

${\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}\mspace{14mu} {and}\text{/}{{or}\mspace{14mu}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{j\; w_{m}} & {{- j}\; w_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from a basestation respectively, and

$\begin{bmatrix}w_{m} \\{j\; w_{m}}\end{bmatrix}\mspace{14mu} {{and}\mspace{11mu}\begin{bmatrix}w_{m}^{\prime} \\{{- j}\; w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from the basestation respectively.

For example, in the terminal 800 according to the eighth embodiment, abasic codebook includes two set of base vectors, and respective basevectors in each set of base vectors are orthogonal with each other. Thecodebook conservation unit 801 of the terminal 800 acquires the vectorsthat are orthogonal with at least one base vector of the second set ofbase vectors, among the acquired vectors computes a vector whosecorrelation with one base vector of the first set of base vectors is thehighest as a new base vector, generates other new base vectors, andreplaces respective original base vectors with respective new basevectors to form a new basic codebook.

According to the present embodiment, the codebook conservation unit 801of the terminal 800 generates other new base vectors from the other basevectors except for the one base vector in the first set of base vectorsby repeating the operation of generating the new one base vector, or bymultiplying the other base vectors except for the one base vector in thefirst set of base vectors with the new one base vector respectively.

According to the present embodiment, the codebook conservation unit 801of the terminal 800 computes linear combinations of at least two basevectors in the second set of base vectors as the vectors beingorthogonal with at least one base vector of the second set of basevectors.

According to the present embodiment, the codebook conservation unit 801of the terminal 800 combines the original basic codebook and the newbasic codebook along the row direction to form an extended codebook. Theterminal 800 further assigns phase offsets to the base vectors in theextended codebook via the phase offset assignment unit 8012 so as toform a codebook with phase offset.

In a case of multiple antenna arrays applied to eight dual-polarizedantennas (each of antenna arrays includes four antennas having the samepolarization feature of the eight dual-polarized antennas), the codebookconservation unit 801 combines the extended codebook and the generatedcodebook with phase offset along the column direction into one codebookas the codebook for the eight dual-polarized antennas.

For example, in the terminal 800 according to the ninth embodiment, acodebook conservation unit 801 stores a basic codebook and/or a newbasic codebook, the basic codebook comprising a plurality of basevectors which are grouped into a plurality of sets, respective basevectors in each set of base vectors being orthogonal with each other,the new basic codebook being obtained in a way of: computating a newbase vector corresponding to one base vector in one set of the pluralityof sets of base vectors, so that the new base vector equals to anotherbase vector in other sets of base vectors other than the one set of basevectors, and repeating the above operation to acquire a plurality of newbase vectors corresponding to the plurality of base vectors, andgenerating the new basic codebook using the respective new base vectors;and a information feedback unit 802 feeds back indexes identifyingpre-coding matrixes in the basic codebook and/or the new basic codebookto a base station.

According to the present embodiment, the plurality of base vectors inthe basic codebook are w_(m), which are DFT vectors, and differencesbetween directions of base vectors w_(m) and w_(m+n) are n times of11.25° in turn, where m and n are integer numbers, and they are alsopossible to be real numbers.

According to the present embodiment, the obtained new base vectors arew′_(m). w′_(m) is also DFT vectors, but the directions of which aredifferent from the corresponding base vectors w_(m). Whereinw′_(m)=w_(m+n), for example, it is possible to have w′_(m)=w_(m+1), orw′_(m)=w_(m+2), or w′_(m)=w_(m+3), etc.

According to the present embodiment, the form of a rank 2 pre-codingvector in the new basic codebook is

${\begin{bmatrix}w_{m} & w_{m}^{\prime} \\w_{m} & {- w_{m}^{\prime}}\end{bmatrix}\mspace{14mu} {and}\text{/}{{or}\mspace{14mu}\begin{bmatrix}w_{m} & w_{m}^{\prime} \\{j\; w_{m}} & {{- j}\; w_{m}^{\prime}}\end{bmatrix}}},$

wherein

$\begin{bmatrix}w_{m} \\w_{m}\end{bmatrix}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}w_{m}^{\prime} \\{- w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from a basestation respectively, and

$\begin{bmatrix}w_{m} \\{j\; w_{m}}\end{bmatrix}\mspace{14mu} {{and}\mspace{11mu}\begin{bmatrix}w_{m}^{\prime} \\{{- j}\; w_{m}^{\prime}}\end{bmatrix}}$

are used to precode the signals in different layers sent from the basestation respectively.

FIG. 13 is a flow chart showing a method of generating a codebook in awireless communication system with the multiple antenna arrays accordingto the present embodiment.

As shown in FIG. 13, at step S1310, a basic codebook including two setsof base vectors in which respective base vectors in each set of basevectors are orthogonal with each other is provided. At step S1320, thevectors being orthogonal with at least one base vector of the second setof base vectors are acquired. At step S1330, a vector whose correlationwith one base vector of the first set of base vectors is the highestamong the acquired vectors is made to be a new base vector. At stepS1340, other base vectors are generated. At step S1350, respectiveoriginal base vectors are replaced with respective new base vectors toform a new basic codebook.

According to the present embodiment, the step of generating the othernew base vectors of the above method of generating codebook may include:repeating the operation of generating the new one base vector withrespect to other base vectors other than the one base vector in thefirst set of base vectors, or multiplying the other base vectors otherthan the one base vector in the first set of base vectors with the newone base vector respectively.

According to the present embodiment, the step of acquiring the vectorsbeing orthogonal with at least one base vector of the second set of basevectors of the above method of generating codebook may further include:computing linear combinations of at least two base vectors of the secondset of base vectors.

According to the present embodiment, the above method of generatingcodebook may further include a step of combining the basic codebook andthe new basic codebook along the row direction to form an extendedcodebook, and assigning phase offsets to the base vectors in theextended codebook to generate a codebook with phase offset.

According to the present embodiment, in the above method of generatingcodebook, the multiple antenna arrays may be eight dual-polarizedantennas and each of antenna arrays includes four antennas having thesame polarization feature of the eight dual-polarized antennas. Themethod may further include a step of combining the extended codebook andthe generated codebook with phase offset along the column direction intoone codebook as the codebook for the eight dual-polarized antennas.

The description of the eighth embodiment of the present disclosure ismade as above. With the technical scheme of the present embodiment, theperformance of the multi-user transmission can be improved withoutreducing the performance of the single-user transmission.

The respective embodiments of the present application as described aboveare only exemplary description, and their specific structures andoperations are not limiting the scope of the present disclosure. Thoseskilled in the art may newly combine various parts and operations in therespective embodiments to produce new embodiments which also accord withthe concept of the present disclosure.

The embodiments of the present disclosure may be implemented byhardware, software or firmware or in a way of combining them with eachother, and the way of implementation is not limiting the scope of thepresent disclosure.

The connection relationships between respective functional elements(units) in the embodiments of the present disclosure are not limitingthe scope of the present disclosure, in which one or more functionalelement(s) may include or be connected to any other functional elements.

Although some embodiments of the present disclosure have been shown anddescribed in combination with the attached drawings, those skilled inthe art should understand that variations and modifications can be madeto these embodiments, which still fall within the claims and theirequivalents of the disclosure, without departing from the principle andspirit of the disclosure.

1. A communication apparatus comprising: a codebook storage which, inoperation, stores an extended codebook, which is generated by combininga first basic codebook and a second basic codebook along a columndirection, wherein the first basic codebook includes N groups eachconsisting of four basic pre-coding column vectors that are orthogonalto each other (N is a natural number equal to or greater than 2), andwherein the second basic codebook is generated by applying phase offsetsto the N groups, respectively, each of the phase offsets applied to theN groups being different from each other; a transmitter which, inoperation, transmits to a base station a pre-coding matrix indicator(PMI) that identifies one pre-coding extended column vector from amongthe 4N pre-coding extended column vectors included in the extendedcodebook; and a receiver which, in operation, receives, from the basestation, downlink data that is pre-coded using the one pre-codingextended column vector identified by the PMI.
 2. The communicationapparatus according to claim 1, wherein each of the first basic codebookand the second basic codebook includes 4N basic pre-coding columnvectors each having four rows and one column; and the extended codebookincludes 4N pre-coding extended column vectors each having eight rowsand one column.
 3. The communication apparatus according to claim 1,wherein an angle difference between the basic pre-coding matrices w_(m)(m is an integer) and w_(m+1) included in the first basic codebook is ntimes 2π/32 (n is an integer).
 4. The communication apparatus accordingto claim 1, wherein in case where the downlink data is transmitted usingtwo layers from the base station apparatus, the basic pre-coding columnvectors included in the first basic codebook respectively include basicpre-coding matrix vectors w_(m) (m is an integer) for one of the twolayers and basic pre-coding matrix vectors w′_(m) for the other of thetwo layers; and an angle difference between the vectors w_(m) and w′_(m)is selected from 2π/32, 4π/32 and 6π/32.
 5. The communication apparatusaccording to claim 1, wherein the base station apparatus comprises aplurality of array antennas; and the transmitter transmits the PMI foreach of the plurality of array antennas.
 6. A communication methodcomprising: storing an extended codebook, which is generated bycombining a first basic codebook and a second basic codebook along acolumn direction, wherein the first basic codebook includes N groupseach consisting of four basic pre-coding column vectors that areorthogonal to each other (N is a natural number equal to or greater than2), and wherein the second basic codebook is generated by applying phaseoffsets to the N groups, respectively, each of the phase offsets appliedto the N groups being different from each other; transmitting to a basestation a pre-coding matrix indicator (PMI) that identifies onepre-coding extended column vector from among the 4N pre-coding extendedcolumn vectors included in the extended codebook; and receiving, fromthe base station, downlink data that is pre-coded using the onepre-coding extended column vector identified by the PMI.
 7. Thecommunication method according to claim 6, wherein each of the firstbasic codebook and the second basic codebook includes 4N basicpre-coding column vectors each having four rows and one column; and theextended codebook includes 4N pre-coding extended column vectors eachhaving eight rows and one column.
 8. The communication method accordingto claim 6, wherein an angle difference between the basic pre-codingmatrices w_(m) (m is an integer) and w_(m+1) included in the first basiccodebook is n times 2π/32 (n is an integer).
 9. The communication methodaccording to claim 6, wherein in case where the downlink data istransmitted using two layers from the base station apparatus, the basicpre-coding column vectors included in the first basic codebookrespectively include basic pre-coding matrix vectors w_(m) (m is aninteger) for one of the two layers and basic pre-coding matrix vectorsw′_(m) for the other of the two layers; and an angle difference betweenthe vectors w_(m) and w′_(m) is selected from 2π/32, 4π/32 and 6π/32.10. The communication method according to claim 6, wherein the basestation apparatus comprises a plurality of array antennas; and thetransmitting includes transmitting the PMI for each of the plurality ofarray antennas.